feat(ui): add the interface size preference (#540)

Preferences > Interface size and the `pref.uiScale` command make the whole
interface larger or smaller. Auto keeps the system's display scale; a fixed
size from 75 % to 300 % is absolute (100 % is one screen pixel per point on
every system). The size is held back while the window is too small for it.

Strings are added in en, es and fr; the control protocol, MCP, UI parity and
roadmap docs are updated.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/control-protocol.md

@@ -169,7 +169,9 @@ instance, a dragged axis slider; see `text.preview` in [mcp.md](mcp.md)).
 `canvas` or `properties`), whether `keyboard` mode is on (F6 or Tab moved the focus since the last
 pointer press, so the focus ring shows) and the name of the `control` with the keyboard (what a
 screen reader announces; empty when nothing has it). `reduceMotion` is true when the preference or
-the system's accessibility setting turns the interface's animations off. `live` is the live
+the system's accessibility setting turns the interface's animations off. `uiScale` is the interface
+size: the `choice` (`"auto"` or a fixed size, `pref.uiScale`), what is `drawn` (physical pixels per
+point) and whether the window is too small for the choice (`limited`, so `drawn` is smaller). `live` is the live
 session of the window's active file as the window shows it (sessions are per file: another tab
 shows its own, or `status: "off"`): `session` (its id, null when the file isn't in one),
 `sessions` (how many the app has running), `status`, `role`, the number of `peers`, who you `follow`, the

docs/mcp.md

@@ -417,6 +417,12 @@ on; `key` resolves keys the way the canvas does. Commands behind keys:
   nested layers), Escape or a click drops it.
 - `pref.reduceMotion {on?}`: reduced motion (the interface's own animations off; the system's
   accessibility setting turns it on too). Prototype transitions still play.
+- `pref.uiScale {scale?}`: how large the whole interface is drawn. `scale` is `"auto"` (the system's
+  display scale, the default) or a fixed size from 0.75 to 3 where 1 is one screen pixel per point
+  whatever scale the system reports, so a 2× screen needs 2 to match Auto. 1.25 means 125 %, not 125
+  (outside the range it stops at the nearest end). Omit `scale` to read it. The canvas zoom is separate.
+  A window too small for a fixed size draws a smaller one until it is larger; the choice is kept.
+  In the browser the interface always follows the browser's zoom.
 - `pref.autoSave {on?}`: files that have a path save themselves a few seconds after they change
   (off by default; the desktop app's timer does it, so scripts still call `file.save`).
 - `pref.language {language: system|en|cs|ru|es|es-419|fr|de|pt-BR|ja|ko|zh-Hans|…}`: the app's interface language, applied live ("system":

docs/ui-parity.md

@@ -165,6 +165,7 @@ in the table. `crates/ui-egui/src/shortcut_keys_tests.rs` drives the keys a look
 | Keyboard navigation: F6 regions, Tab within, focus ring | done | |
 | Layers panel from the keyboard: arrows, F2 / Cmd+R rename (the row gets the keyboard back), Shift+F10 context menu; 10k-layer lists | done | revealing a layer, expanding, collapsing and arrow keys timed in `cargo xtask bench` |
 | Reduced motion | done | system setting or preference |
+| Interface size (Preferences ▸ Interface size, `pref.uiScale`) | done on desktop | Auto (the system's scale) or a fixed size from 75 % to 300 %, where 100 % is one screen pixel per point on every system; held back while the window is too small for it; the browser keeps its own zoom |
 | WCAG AA contrast of secondary and accent text | done | |
 | Tested with VoiceOver, NVDA, Narrator, Orca | missing | human QA |
 
@@ -176,6 +177,7 @@ in the table. `crates/ui-egui/src/shortcut_keys_tests.rs` drives the keys a look
 | 2026-10-17 | minor | Escape or Enter while the pen draws ends the path and keeps its vector in edit mode with the pen, so the next path is drawn into the same layer (unconnected); another Escape leaves edit mode (#760). Entering edit mode on a layer that already is a vector no longer records an empty undo step. Numbers unchanged |
 | 2026-10-17 | minor | Text layers keep the platform IME on for every frame of an edit, not only frames with input (an idle frame turned it off and macOS dropped each composition, #747); the candidate window opens at the caret instead of the canvas corner; keys pressed while composing go to the IME |
 | 2026-10-17 | minor | The community batches #733, #735, #736, #729 and #753: submenus open after a short hover, keep their place along a safe triangle and reopen closed (main and context menus); the Actions palette's tabs, ranked search, keyboard highlight and recents; menu items that need a value ask for it, File ▸ Rename edits in place; Shift+I and Resources open Assets; a draggable Pages/Layers divider; Shift-drag moves the selection; a double-click near a thin stroke, merging dropped points and Flatten keeping open paths in vector editing; live font preview and the selection highlight after line breaks; the colour picker on a new fill or stroke; Paste over and Paste properties from the keyboard; Selection colors (partial: no library styles or variables there) and smart-selection spacing fields and canvas handles (partial: no reorder rings), both also checked by their author on macOS; the keyboard shortcuts panel closes on the home screen; Adwaita window controls on Wayland compositors without server decorations (checked by its author on GNOME; KDE not run). Checked in the app on Windows (submenus by their author), not side by side with Figma. Summary and hours unchanged |
+| 2026-10-17 | minor | Interface size preference (#540): Preferences ▸ Interface size and `pref.uiScale`, Auto or a fixed size from 75 % to 300 %, absolute (100 % is one screen pixel per point whatever the system's scale) |
 | 2026-10-15 | minor | Hex fields take CSS colour names (`red` → `FF0000`): fills, strokes, effects, grids, variables, backgrounds and the colour picker |
 | 2026-10-15 | minor | Typography: drag the line-height, letter-spacing and paragraph-spacing icons to scrub their values; shared numeric-field rates, resize cursor and one Undo step per gesture (#373). |
 | 2026-10-15 | minor | A click or Tab into a one-line field selects all of its text (hex, inline, name, search and dialog fields); the first click into a fill's hex takes the keyboard |