Add Finnish interface language

Adds a Finnish (Suomi, `fi`) catalog with 4,770 entries, written in
sentence case from the English labels with ordinary Finnish
vector-illustration vocabulary. It covers menus, `tl!` literals and the
668 `@msg` status and error messages: `fi` is in KEEPS_MENU_NAMES and
COMPLETE_MESSAGES. Like the other complete catalogs, it leaves the
product, workspace and perspective preset names in English.

Plurals are one/other with the partitive singular after any number but
1 (1 taso, 2 tasoa). Any fi-* locale selects Finnish; sv-FI does not.

Tests: `cargo test -p vectorcraft-ui-egui --lib i18n` passes (47),
including the new finnish_reads_as_finnish and the bundled-font glyph
check.

Clean-room, AI-assisted (Claude Code) translation; a native speaker
checked it in the running app, but it has not been proofread line by
line.

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

docs/development.md

@@ -194,7 +194,7 @@ always use the English ids and labels, so agents and scripts never see translate
 Languages shipped: English (`en`, the source), Traditional Chinese (`zh-hant`, complete, in the vocabulary used
 in Taiwan; `zh-TW`, `zh-HK`, `zh-MO` and `zh-Hant-*` locales all resolve to it), Simplified Chinese (`zh-hans`,
 complete, in the vocabulary used in mainland China; `zh-CN`, `zh-SG`, `zh-Hans-*` and a bare `zh` resolve to it,
-so the two scripts never mix), Japanese (`ja`, complete), Korean (`ko`, complete; every `ko-*` locale resolves to it; a count takes no plural, and Hangul is drawn with an installed system font), Vietnamese (`vi`, complete; every `vi-*` locale resolves to it; a count takes no plural, so plurals have one form), German (`de`, complete; `de-DE`, `de-AT`, `de-CH` and every
+so the two scripts never mix), Japanese (`ja`, complete), Korean (`ko`, complete; every `ko-*` locale resolves to it; a count takes no plural, and Hangul is drawn with an installed system font), Vietnamese (`vi`, complete; every `vi-*` locale resolves to it; a count takes no plural, so plurals have one form), Finnish (`fi`, complete; every `fi-*` locale resolves to it; one/other plurals with the partitive after numbers other than 1), German (`de`, complete; `de-DE`, `de-AT`, `de-CH` and every
 other `de-*` locale resolve to it), Spanish (`es`, complete, in neutral
 international Spanish; every `es-*` locale such as `es-ES`, `es-MX`, `es-AR` or `es-419` resolves to it), French (`fr`,
 complete; `fr-FR`, `fr-BE`, `fr-CA`, `fr-CH` and every other `fr-*` locale resolve to it), Italian (`it`,
@@ -231,7 +231,7 @@ other `sv-*` locale resolve to it), Czech (`cs`, every menu label) and Brazilian
   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). Czech, German, Spanish, French, Hungarian, Italian, Japanese, Korean, Polish, Brazilian Portuguese, Russian, Swedish, Ukrainian and Vietnamese cover every
+  placeholders are translated in turn (the reason after `: {e}` is often a message too). Czech, German, Spanish, French, Hungarian, Italian, Japanese, Korean, Polish, Brazilian Portuguese, Russian, Swedish, Ukrainian, Vietnamese and Finnish 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 a row in each of those catalogs (`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