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/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.