Export: Save for Office, CSS and Copy warn when an image couldn't be decoded or a font is missing

Save for Office Documents, the pictures that css.export rasterizes and
the PNG and PDF that Copy offers other apps were encoded with
fileio::encode, which drops the encoder's warnings. When the art held
an image that the renderer could not decode, the PNGs lacked it; type
in a font that is not installed came out in the fallback font in the
Office PNG and Copy's PNG and PDF. No reply or status message said so.
The app also dropped the warnings that document.exportSelection
returns.

document.exportForOffice, clipboard.exportPng and clipboard.exportPdf
now return `warnings`, and css.export adds the notes of its pictures to
its `warnings`, each once. Save for Office Documents and
document.exportSelection show the first note in the status bar in Save
As's message, and the CSS Properties panel's Export Selected CSS… and
Export All… show it in Export As's message. After Copy or Cut, once
the PNG and PDF are on the system clipboard, the status bar shows their
first note. The files are written as before.

Docs: docs/mcp.md (Clipboard, CSS Properties), docs/control-protocol.md.

docs/control-protocol.md

@@ -116,13 +116,13 @@ the two can still win that race.
 | `app.export` JPEG options | `{path?, format: jpg, quality?: 0–100, colorModel?: rgb \| cmyk \| gray, method?: baseline \| optimized \| progressive, scans?: 3–5, embedIcc?, imageMap?: none \| client \| server}` | `cmyk` writes ink amounts in the working CMYK space (CMYK colours keep their inks; RGB ones are separated with the colour settings); `embedIcc` (default true) embeds sRGB, the working CMYK profile or a grey profile, generated by the CMS; `imageMap` also writes `<stem>.html` (client-side `<map>`) or `<stem>.map` (NCSA) beside the image for the objects with a URL and an Image Map shape (`attributes.set`), returned in `linked`. The `jpgOptions` dialog has these fields too (`quality` is shown 0–10; a CMYK document defaults to `cmyk`) |
 | `app.export` PNG-8 and GIF | `{path?, format: png8 \| gif, colors?: 2–256, reduction?: perceptual \| selective \| adaptive \| web \| blackWhite \| gray, dither?: none \| diffusion \| pattern \| noise, ditherAmount?: 0–100, transparency?, matte?, interlaced?, …raster options}` | a palette of at most `colors` entries (art with that many colours or fewer keeps them exactly); pixels under half opacity become one transparent entry unless `transparency` is false, partly transparent edges are blended over `matte` (`none` keeps their colour). `png8` writes an indexed `.png` (1–8 bits per pixel); Export for Screens rows take both. The `png8Options` / `gifOptions` dialogs have these fields |
 | `app.export` Text and WebP | `{path?, format: txt, encoding?: utf8 \| utf16, lineEndings?: lf \| crlf, selectionOnly?}` / `{format: webp, lossless?, quality?}` | Text writes the stories in stacking order (back to front; a thread once; hidden objects and template layers left out), each line ended by `lineEndings`; `utf16` is little-endian with a byte order mark. Export As → Text opens `txtOptions` (fields `path`, `encoding`, `lineEndings`, `selectionOnly`; OK writes the file). WebP is always lossless: `lossless: false` adds a warning |
-| `engine.execute` `document.exportForOffice` | `{}` / `{path?, ppi?: 72 \| 150 \| 300, transparent?, artboard?}` | File → Save for Office Documents…: without params opens `saveForOffice` (fields `ppi`, `transparent`, `artboard`; OK picks the file); with params writes the artboard as a PNG (`path`, else a picked file) → `{path}`. Headless it returns `{dataBase64, width, height}` |
+| `engine.execute` `document.exportForOffice` | `{}` / `{path?, ppi?: 72 \| 150 \| 300, transparent?, artboard?}` | File → Save for Office Documents…: without params opens `saveForOffice` (fields `ppi`, `transparent`, `artboard`; OK picks the file); with params writes the artboard as a PNG (`path`, else a picked file) → `{path}`, and the status bar shows the first of the export's `warnings` as Save As shows a save's notes. Headless it returns `{dataBase64, width, height, warnings}` |
 | Native file dialogs | any command with no `path` that needs one (`file.open`, `file.saveAs`, `file.place`, exports…) | on macOS (a sheet on the window) and Linux the dialog doesn't hold the window: the call returns `"cancelled"` at once, `ui.inspect` reports `fileDialog`, and when the person at the computer picks a file the command runs again on the document it was asked for. While it is open another dialog is refused: that call returns `"cancelled"` too, and the status bar (`ui.inspect` → `ui.status`) says `"A file dialog is open: choose there first"`; on macOS `app.quit`, `file.close` and `file.closeAll` return that message and the menu bar does nothing (#592, #867). On Windows the dialog is shown in line: the call returns once it closes. Pass `path` to skip the dialog |
 | Background Save / Export | `app.save`, `app.export {path}`, `file.save`, `file.saveAs`, `file.saveCopy` | with the `backgroundSave` / `backgroundExport` preferences on (the default) the desktop app writes the file from a snapshot on a worker thread: the call returns `{path, background: true, status}` at once, `ui.inspect` lists the jobs still running in `background` (labels such as `"Saving a.vectorcraft"`), and the document counts as saved (as of the snapshot) once its file is written. Quit and Close wait for them. With the preferences off (`prefs.set`), saves return once written |
 | `engine.execute` `file.exportForScreens` | `{}` / `{…document.exportForScreens params}` | without params opens `exportForScreens` on the settings the document last exported with (`document.exportSettings`). While the Artboard tool is in use, the artboards it has selected (its `selected` option, the rows the Artboards panel highlights) are checked instead, and `select` is `range`, or `all` when every artboard is selected. Fields: `tab` (`artboards` \| `assets` \| `layers` \| `selection`; OK reads Export Layer or Export Selection on those), `layers` ([layer id…] checked), `selectionMode` (`one` \| `each`), `name` (the file name pattern of `document.exportForScreens`), `counterStart`, `folders` (`none` \| `rows` \| `format` \| `item` \| `itemFormat`), `exportPreset` (the preset applied last), `__savePreset` (the name typed to save one), `select` (`all` \| `range` \| `full`), `range` ("1-3, 5"), `boards` (a bool per artboard; checking them sets `select` and `range`), `includeBleed`, `folder` (desktop; default the Desktop, else home), `openLocation`, `subfolders`, `prefix`, `preset` (`""` \| `mobile` \| `density`), `formats` (rows `{format, scale: "2x" \| "100w" \| "100h" \| "72ppi", suffix, quality?, folder?}`), `settings` (`{png \| png8 \| jpg \| webp \| gif \| svg \| pdf: {…options}}`, the gear's Format Settings; `__settings` names the format shown). `ui.dialog.confirm` exports (a bad range or no folder keeps it open); on the web OK reads "Download" and the files download (several as one `.zip`). With params it runs `document.exportForScreens`: no `folder` returns the files (or with `zip: true` one store-only zip) as `dataBase64`; with `openLocation` the first file is shown in the file manager. When `warnings` is not empty, an export into a folder or a download shows the first in the status bar as Save As shows a save's notes |
 | Export for Screens' Assets tab, Asset Export panel | `ui.dialog.set` `tab: "assets"`, `assets: [asset id…]`; `window.panel {panel: "assetExport"}` | on the Assets tab of `exportForScreens` the `assets` field lists the checked asset ids (all of them when it opens); `ui.dialog.confirm` runs `document.exportForScreens {assets}` (nothing checked keeps it open) and OK reads "Export Asset". The Asset Export panel acts through `assets.*` and shares the format rows with the dialog (`assets.settings.set`) |
 | `engine.execute` `file.exportSelection` | `{multiple?}` | File › Export Selection…: collects the selected objects as assets (`assets.add`; one per object unless `multiple` is false) and opens `exportForScreens` on its Assets tab with just them checked → `{assets}`. `document.exportSelection` still exports the selection in one go, without the dialog |
-| CSS Properties panel | `window.panel {panel: "cssProperties"}`; `engine.execute` `css.copy` / `css.exportFile` | the panel shows `css.selection` of the selection (after Generate CSS, while nothing is selected, `css.generate`); its menu holds the options the commands take (`units`, `position`, `dimensions`, `unnamed`, `rasterize`). `css.copy {scope?, …options}` copies the CSS to the clipboard → `{css, rules}`; `css.exportFile {path?, scope?, …options}` writes `css.export` to `path` (else a picked file; the web downloads it) with the pictures of rasterized art next to it → `{path, rules, images}` |
+| CSS Properties panel | `window.panel {panel: "cssProperties"}`; `engine.execute` `css.copy` / `css.exportFile` | the panel shows `css.selection` of the selection (after Generate CSS, while nothing is selected, `css.generate`); its menu holds the options the commands take (`units`, `position`, `dimensions`, `unnamed`, `rasterize`). `css.copy {scope?, …options}` copies the CSS to the clipboard → `{css, rules}`; `css.exportFile {path?, scope?, …options}` writes `css.export` to `path` (else a picked file; the web downloads it) with the pictures of rasterized art next to it → `{path, rules, images, warnings}`, and the status bar shows the first of `warnings` as Export As shows an export's notes |
 
 | `ui.dialog.*` on `importPdf` | `{field: "page" \| "allPages" \| "range" \| "cropTo" \| "password", value}` | opening (`app.open`, `file.open`) a PDF or `.ai` with several pages or a password, or placing one (`file.place` without `page`), opens the Import PDF dialog (`mode`: `open` or `place`; `pageCount`). While `__locked` is true, set `password` and `ui.dialog.confirm` to unlock it (a one-page file then opens or is placed). Then set `allPages: false` and `range` ("2-3, 5") to open some pages, or `page` to place one, and `cropTo` (`bounding`, `art`, `crop`, `trim`, `bleed`, `media`); `ui.dialog.confirm` opens or places them (placing keeps the other `file.place` params, such as `link`, `template` and `at`, in `__place`). `document.open {pages, cropTo, password}` and `file.place {page, crop, password}` do the same without the dialog |
 
@@ -572,7 +572,8 @@ dialog, a drop) opens the `textImport` dialog (fields `platform`: `windows`/`mac
 type with those options (`file.place {text: {…}}`).
 
 System clipboard formats: through the desktop app, `edit.copy` and `edit.cut` publish what `clipboard.flavours`
-lists to the system clipboard, and every Paste command first loads what another app copied (`clipboard.importSvg`,
+lists to the system clipboard (once they are written, the status bar shows the first of the PNG's and PDF's
+`warnings`, such as an image that couldn't be decoded), and every Paste command first loads what another app copied (`clipboard.importSvg`,
 `importPdf`, `importEps`, `importText` or `importImage`, centred in the view; the desktop app reads PDF and EPS from
 the Windows clipboard and, with SVG, from the macOS pasteboard, and an `.eps` file, or a `.ai` or `.ait` file that holds a PDF or PostScript, copied in a file manager anywhere); an import error answers the Paste with that error
 and pastes nothing. Cmd+V also pastes a bitmap or PDF with no text beside it (on the key's release, as egui sends no