Add French (fr) locale with polite vous form

[AdaInTheLab]

Summary

Second of three planned language additions (German shipped in #59, Spanish queued next). French, polite vous form throughout.

What's in the locale

• Full mirror of `en.ts` (~250 keys across 30 namespaces) translated to French
• Polite form (`vous`) — appropriate register for an admin tool
• Technical proper nouns kept in English: Steam, EAC, telnet, Cloudflare, Discord, IANA, BBGGRR, etc.
• Game-specific terms use the conventional 7D2D-FR renderings: « Lune de sang », « Z-Kills », etc.
• Action buttons in standard French UI infinitive form (Enregistrer, Modifier, Supprimer, ...)
• Browser auto-detect extended — visitors with `fr-*` locale (fr-FR, fr-CA, fr-BE, etc.) land on French automatically
• broadcastsHint translated to French (the language-switcher tooltip flagging that in-game broadcasts don't auto-translate)

Quality note

This is a competent first pass, not a professional localization. Common UX surfaces should read naturally; the deeper config-editor field labels are direct translations and would benefit from a domain-fluent native-speaker polish. PRs welcome.

Files

File	What
`frontend/src/i18n/locales/fr.ts`	New
`frontend/src/i18n/index.ts`	Register `fr`, add to `SUPPORTED_LOCALES`, extend browser-language detect

Test plan

• Pull, build, deploy
• Hard-refresh panel → language dropdown shows "Français" between "Deutsch" and "简体中文"
• Switch to Français — sidebar nav, dashboard, settings, etc. render in French
• Hover the language switcher → tooltip shows the broadcasts-not-translated note in French
• Set browser language to `fr-FR`, clear localStorage, reload → defaults to Français
• vue-tsc passes (structural typecheck across all 7 locales)

Next up

• Spanish (es.ts) — separate PR, same pattern with polite `usted` form

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