Save As: on Linux the file type picked in the save dialog decides the format

On Linux the save dialog is the XDG desktop portal's, shown through
rfd. rfd sends no `current_filter` and reads only the file name from
the portal's answer, and the GTK file chooser keeps the typed name
when another file type is picked. With "PDF-compatible .ai" picked,
`poster` typed for a new document was saved as a native
`poster.vectorcraft`, and so was the suggested
`Untitled-1.vectorcraft` left as it was.

When a save offers two or more file types (Save As, Save a Copy, Save
as Template and the first Save of a new document), the app now calls
the portal's SaveFile itself over D-Bus. The dialog starts on the
suggested file's type, and its answer names the type picked. A name
typed without an extension, or the suggested name left as it was,
gets that type's extension. A name typed with another save format's
extension, other than the suggested file's (for Save as Template, a
template format's), is saved in that format, and any other extension
gets the picked type's added. The dialog keeps the window as its
parent on X11 and Wayland, and rfd shows the dialog when the portal
can't. The same code runs on the BSDs; Windows, macOS and the web
build keep their dialogs.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -690,6 +690,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - `.vectorcraft` (lossless JSON, compressed, atomic saves, save down to v1), Save As with a format chooser, Save a Copy, Revert, templates, Data Recovery after a crash (browser tabs hold their copies with Web Locks too, so a background tab's copies are never taken for a crash's, and a tab repairs copies another removed when it resumes: #367), background save and export. The desktop app writes its warnings and errors (graphics adapter, a lost graphics device, panics the guard recovered from) to standard error and to `logs/vectorcraft.log` next to the preferences, the two previous runs kept and each file capped at 16 MiB, so a launch without a terminal still leaves something to attach to a bug report (#465). Native documents with a lot of path data (traced or generated art, in layers, symbols or patterns) are saved as compact JSON, about a fifth of the size; small documents stay pretty-printed (#1492). Saving to a network drive on macOS falls back to a plain fsync when the share refuses a full flush (#1625, part of #1620; not confirmed on a real share), and a save still succeeds when a folder refuses the temporary file, the rename over the document or copying permissions: the content is written elsewhere first and copied in, and a failed copy keeps the temporary file and names it (#1649, part of #1620). A document opened by a relative path finds its relatively linked images, and `vectorcraft-cli convert` lists what reading the input left out as `importWarnings` (#1606, fixes #1604).
     A name typed in the desktop save panel without the file type's extension gets it, and when a file of that name exists, which the panel did not ask about, Save, Save As, Save a Copy, Save as Template, the exports and every other write through a save panel ask before replacing it (dialog `replaceFile`; Cancel writes nothing).
   - macOS: the save sheet of Save As, Save a Copy, Save as Template and the first Save of a new document has a Format menu that lists each format the save can write, with the suggested file's format selected. Choosing a format changes the extension in the name field, and a name typed without an extension gets the chosen format's. A name typed with another listed extension is saved in that format. The menu is set up before the sheet opens; a save panel with one file type is shown by rfd as before (#866, `apps/vectorcraft/src/mac_save_format.rs`).
+  - Linux and the BSDs: the save dialog of Save As, Save a Copy, Save as Template and the first Save of a new document is asked of the desktop portal, starting on the suggested file's type, and the file type picked in it decides the format. A name typed without an extension, or with the suggested file's, gets the picked type's extension; a name typed with another save format's extension (for Save as Template, a template format's) is saved in that format, and any other extension gets the picked type's added. rfd shows the dialog when the portal can't (#2679, `apps/vectorcraft/src/portal_save.rs`).
   - Save as Template writes a PDF-compatible `.ait` as well as a `.vctemplate`, which stays the default: the save panel and the web build's Save as Template dialog list both, and New from Template opens the `.ait` as an untitled document with its layers and art (`file.saveAsTemplate {format: "ait"}`, or a path ending in `.ait`).
     Save as Template always writes a file named with its template type's extension: `file.saveAsTemplate` saves a path ending in another format's extension (`card.png`) as `card.vctemplate` (or `card.ait`), the desktop save panel adds `.vctemplate` to a name typed with a Save As format's extension (`card.pdf.vctemplate`), and `file.saveAs` and `document.save` refuse `ait` with an error that names `file.saveAsTemplate`. A `.ait` is PDF-compatible only: `aiVersion: "7"` is an error for it, and the web build's dialog shows no Version row for it.
     Save as Template always writes a file named with its template type's extension: `file.saveAsTemplate` saves a path ending in another format's extension (`card.png`) as `card.vctemplate` (or `card.ait`), the desktop save panel adds `.vctemplate` to a name typed with a Save As format's extension (`card.pdf.vctemplate`), and `file.saveAs` and `document.save` refuse `ait` with an error that names `file.saveAsTemplate`.