French (fr) UI localization (#1224)

Adds a French catalog as a partial locale (missing strings fall back to English, like Russian), registered in the language list, with tests and attribution.

docs/control-protocol.md

@@ -127,7 +127,7 @@ unit regardless of the design, pass an expression with one: `"value": "50 mm"` o
 | `ui.home` | `open?`: bool; `sample?`: index | the start page: show or hide it, open a built-in sample (once built); returns `open` and the recent designs |
 | `ui.shortcut` | `command`, `key?`, `replace?` | a command's key, or rebinds it (`""` clears; a key another command uses is refused unless `replace`) |
 | `ui.help` | `item?`: `about`, `shortcuts` (read only), `report` (opens a report draft for review), `crashes` (the Crash Reports window) | the version, commit, build date and diagnostic text |
-| `ui.prefs` | any of `default_units`, `nav` (`fusion`, `solidworks`, `inventor`, `alias`, `tinkercad`, `openscad`), `zoom_reverse`, `zoom_to_cursor`, `smooth_wheel_zoom` (default true), `trackpad_pan` (default true: two-finger scroll pans, pinch zooms), `orbit_cursor`, `look_at_anim`, `look_at_speed`, `middle_double_click` (`home` (default), `fit`, `off`), `msaa`, `length_decimals`, `angle_decimals`, `theme` (`dark`, `light`, `system`), `language` (`en`, `ja`, `de`, `ko`, `ru`, `zh`; see [localization](localization.md)), `ui_scale`, `up_axis` (`z`, `y`), `offer_stand_up`, `color_vision` (`off` (default), `protan`, `deutan`, `tritan`: colour vision modes, #778), `check_updates` (default true; [updates](book/src/guide/updates.md)); `open` (a page: General, Navigation, Keyboard Shortcuts, Design, Graphics, Sketch, Selection) opens the Preferences dialog there; `button`: `ok`, `cancel`, `apply`, `defaults`; `stand_up: true` stands up a just-opened Y-up file | sets preferences (kept between runs), returns them all plus `dialog` (the open page or null), `dark` and `stand_up_offer` |
+| `ui.prefs` | any of `default_units`, `nav` (`fusion`, `solidworks`, `inventor`, `alias`, `tinkercad`, `openscad`), `zoom_reverse`, `zoom_to_cursor`, `smooth_wheel_zoom` (default true), `trackpad_pan` (default true: two-finger scroll pans, pinch zooms), `orbit_cursor`, `look_at_anim`, `look_at_speed`, `middle_double_click` (`home` (default), `fit`, `off`), `msaa`, `length_decimals`, `angle_decimals`, `theme` (`dark`, `light`, `system`), `language` (`en`, `ja`, `de`, `fr`, `ko`, `ru`, `zh`; see [localization](localization.md)), `ui_scale`, `up_axis` (`z`, `y`), `offer_stand_up`, `color_vision` (`off` (default), `protan`, `deutan`, `tritan`: colour vision modes, #778), `check_updates` (default true; [updates](book/src/guide/updates.md)); `open` (a page: General, Navigation, Keyboard Shortcuts, Design, Graphics, Sketch, Selection) opens the Preferences dialog there; `button`: `ok`, `cancel`, `apply`, `defaults`; `stand_up: true` stands up a just-opened Y-up file | sets preferences (kept between runs), returns them all plus `dialog` (the open page or null), `dark` and `stand_up_offer` |
 | `ui.capture` | `action?`: `capture`, `revert`, `cancel` | answers the Capture Position question (asked when a command starts or the design is saved while components were moved without capturing); without `action`, what waits |
 | `ui.window` | `action`: minimize, maximize, restore, toggle, close | what the title bar's caption buttons do (on Windows and Linux the application bar is the title bar); close asks about unsaved designs first |
 | `ui.agent_cursor` | `show` (bool), `speed`: a number from 1 to 5 (times the normal speed) or `"instant"` (no animation: the command runs at once and the pointer jumps to where it acted), `follow_camera` (bool); all optional | the settings. While the cursor is shown, `engine.execute` first glides to the command's toolbar button and to each pick, then runs and replies; later requests wait their turn. `"animate": false` on a call skips it |

docs/localization.md

@@ -1,7 +1,7 @@
 # UI localization
 
-SolidCraft embeds its translations and works offline. English, Japanese, German, Korean, Russian and
-Chinese are currently registered in `crates/ui-egui/src/i18n.rs`. Adding a language requires an
+SolidCraft embeds its translations and works offline. English, Japanese, German, French, Korean, Russian
+and Chinese are currently registered in `crates/ui-egui/src/i18n.rs`. Adding a language requires an
 embedded catalog and one registration there; the preferences selector and validation use the same
 registry.
 
@@ -31,9 +31,9 @@ that looks untranslated. Two guards catch that in the build:
 
 - `every_complete_catalog_carries_every_english_message` — adding a key to `en.json`
   without giving it to a complete catalog fails the tests. English is the source catalog,
-  Japanese and Chinese are delivered complete; German, Korean and Russian are partial — their
+  Japanese and Chinese are delivered complete; German, Korean, Russian and French are partial — their
   gaps fall back to English and are reported by `cargo xtask i18n` without failing the build. A
-  new label needs en, ja, zh, de and ko; Russian may follow later.
+  new label needs en, ja, zh, de and ko; Russian and French may follow later.
 - `every_toolbar_tab_and_panel_is_in_the_english_catalog` — tabs and panels reach the UI as bare
   source strings, resolved only by the English-label adapter, so a heading missing from `en.json`
   can never translate.