feat: add French translation and enable the fr locale

[BryanFRD]

What this adds

French locale for SteelDocs, at full parity with the English content.

• src/content/docs/fr/ — all 28 documentation pages
• src/content/i18n/fr.json — the 17 UI strings, mirroring en.json
• src/pages/fr/ — index, download, tracker, mirroring the de and es pages
• astro.config.ts — fr keys in the title and sidebar translations maps, and the
  fr entry in locales

fr is enabled rather than commented out like de and es, because the translation is
complete. That alone makes both language switchers appear; no switcher code was touched. If
you would rather review the content first, commenting out those four lines splits this in two.

Terminology

Kept in English: chunk, item, seed, block entity, feature flag, wildcard, and all
identifiers, permission keys, TOML keys, paths and code. Translated: block -> bloc, world ->
monde, domain -> domaine, world generator -> générateur de monde. Benchmark figures use French
number formatting; the chart SVGs are shared assets and stay in English.

How this was tested

bun run build passes: 114 pages, no errors or warnings. All 31 French routes render with
translated content rather than the English fallback. Anchors and visual layout are not checked
by the build, so /fr/configuration/world-configuration and /fr/commands/permissions are
worth a look since they carry the most cross-page anchor links.

One thing to flag

With a second locale active, the build now emits 18 routes under /fr/de/... and /fr/es/....
That happens because src/content/docs/de/ and src/content/docs/es/ are not registered
locales, so Starlight treats them as ordinary root-locale content and the fallback duplicates
them under every active locale. It is not caused by this PR, but this PR is what makes it
visible. Enabling de and es, or moving their content out of the docs tree until they are
ready, would both fix it.

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[coco875]

4.7k, a lot of change to see at once x)