French interface translation

[jo2904]

Adds French as an interface language.

What changes

• Catalogues: translations/messages.fr.yaml covers all 3,118 keys of the English catalogue, plus validators.fr.yaml and security.fr.yaml. CatalogueParityTest now checks French alongside German and pirate English.
• Registration:
  • AppLocale::French (fr, « Français », 🇫🇷) is added, and fr joins framework.enabled_locales.
  • Both language pickers (Settings → General and the first-run install) read AppLocale::cases(), so French shows up there without further changes.
• Clock: French defaults to the 24-hour clock, as German does (ClockFormat::forLocale()); a test is added.
• Emoji picker: assets/emoji/fr/data.json, so emoji search works with French names.
  • It comes from the same emoji-picker-element-data release as en/ and de/; I checked that those two are byte-identical to the published ones.
  • The template now picks de, fr or en instead of testing only for de.
• Docs: the README and features/appearance.md now list the language (German translation and its hash updated), and the CHANGELOG has an entry under a new Unreleased section.

Translation choices

• Formal register. It uses « vous », the usual register for French software; the German catalogue uses « du », and no test enforces either for French.

• Terminology:

  English	French
  calendar	agenda
  label	libellé
  star	suivi (as in Gmail's French UI)
  snooze	mettre en attente

  Google and Microsoft console labels are written as their French UIs show them.

• Typography:

  • The typographic apostrophe (’) is used throughout, so single-quoted YAML never needs '' escaping.
  • Values containing the French spaced colon (« : ») are quoted, because YAML rejects them unquoted.

• Checks: every key's placeholders (%x%, {x}, {{ x }}) and plural forms match the English. All three files load through Symfony's YAML loader, and debug:translation fr --only-missing reports nothing.

Not in this PR

• DeterministicDateDetector still reads dates in mail content in English and German only; French dates (« le 4 août ») are a separate change.
• Keys added by the open PRs Connect my Nextcloud: one-click CalDAV connection when NEXTCLOUD_URL is set #108 and CardDAV: mirror address books, suggest contacts while composing, edit with two-way sync #109 will need French entries once those land. CatalogueParityTest will point them out.

Testing

Run here without Postgres:

• translation parity and catalogue parsing;
• documentation tests;
• ClockFormatResolverTest;
• PHPStan on the changed PHP;
• lint:twig.

EmbeddingPresetTest, which boots the translator for every locale including fr, and the clock rendering tests need the database stack.

🤖 Generated with Claude Code

https://claude.ai/code/session_018nYWPc4bWacb79hm4ytfni

[karatektus]

Merged, thank you — the catalogues are excellent. On top of it I added French to the content readers rather than leaving it for later: the calendar date detector (months, weekdays, 1er, 14h30, demain/après-demain, mardi prochain, French quote attributions), day-first slashed dates for French readers, and French vocabulary in the invoice and login-code extractors (b4b4eb1).