Add French (fr) interface translation

Add fr.tsv with the full interface coverage: every menu string, every
tl! literal, the plurals and the status/error messages (@msg).
Registered as complete_menus with the French plural rule (0 and 1 are
singular); every fr-* locale (fr-FR, fr-BE, fr-CA, fr-CH …) resolves
to it. Keeps the product, workspace and perspective preset names in
English like the other complete catalogs.

Tests: french_reads_as_french; fr added to KEEPS_MENU_NAMES and
COMPLETE_MESSAGES; the Latin-script glyph test covers French letters
and the no-break space; locale tests use de-DE as the unsupported case.

Clean-room: written from the meaning of the English labels in the
vocabulary French-speaking users of vector illustration software know
(Plan de travail, Nuancier, Contour, Fond, Tracé, Calque). No other
application's localisation was copied.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

docs/development.md

@@ -136,7 +136,8 @@ complete, in the vocabulary used in mainland China; `zh-CN`, `zh-SG`, `zh-Hans-*
 so the two scripts never mix), Japanese (`ja`, complete), Spanish (`es`, complete, in neutral
 international Spanish; every `es-*` locale such as `es-ES`, `es-MX`, `es-AR` or `es-419` resolves to it), Italian (`it`,
 complete; `it-IT`, `it-CH` and every other `it-*` locale resolve to it), Russian (`ru`, complete; every `ru-*`
-locale such as `ru-RU`, `ru-BY` or `ru-KZ` resolves to it), Czech (`cs`, every menu label) and Brazilian Portuguese
+locale such as `ru-RU`, `ru-BY` or `ru-KZ` resolves to it), French (`fr`, complete; `fr-FR`, `fr-BE`,
+`fr-CA`, `fr-CH` and every other `fr-*` locale resolve to it), Czech (`cs`, every menu label) and Brazilian Portuguese
 (`pt-br`, every menu label and every `tl!` literal). Untranslated text falls back to English until its rows are added.
 
 - `tl!("…")` translates a literal into the language the UI is drawn in; `i18n::t(s)` is the same for a
@@ -164,10 +165,10 @@ locale such as `ru-RU`, `ru-BY` or `ru-KZ` resolves to it), Czech (`cs`, every m
   control channel, MCP and tests read them) and are translated only where the status bar draws them
   (`i18n::msg`). `@msg` catalog rows hold a whole message or a template such as
   `Couldn't open {name}: {e}`; `{_1}`, `{_2}` … stand for the format string's `{}`, and the values in the
-  placeholders are translated in turn (the reason after `: {e}` is often a message too). Spanish, Italian and Russian cover every
+  placeholders are translated in turn (the reason after `: {e}` is often a message too). Spanish, Italian, Russian and French cover every
   message literal the test scan finds (`complete_languages_translate_every_message`, languages listed in
   `COMPLETE_MESSAGES`): a new `Err("…")`, `Other(…)`, `#[error(…)]` or `status(…)` message needs an `es.tsv`,
-  an `it.tsv` and a `ru.tsv` row (`VECTORCRAFT_I18N_DUMP_MESSAGES=messages.txt cargo test -p vectorcraft-ui-egui
+  an `it.tsv`, a `ru.tsv` and a `fr.tsv` row (`VECTORCRAFT_I18N_DUMP_MESSAGES=messages.txt cargo test -p vectorcraft-ui-egui
   complete_languages_translate_every_message` lists them all). Other languages show messages in English
   until they add `@msg` rows.
 - Not translated on purpose: names that are user data (layers, swatches, fonts, documents), the tab