feat(frontend): add regional English locales

[hmans]

Why

Users cannot explicitly choose an English regional variant, so the generic en locale mixes regional wording and can hide formatting differences.

What changed

• Makes en-GB the canonical locale, adds sparse en-US wording overrides, and exposes both choices in Display settings.
• Migrates browser-local en preferences to en-GB, preserves exact regional negotiation and document metadata, and keeps regional date/time conventions authoritative.
• Generalizes generated locale loading and production chunks from Inlang configuration, with the US override remaining lazy-loaded.
• Updates ADR-043, FDR-022, contributor guidance, and regression coverage for the regional-locale policy.

Test plan

• mise test-frontend — 246 files and 2,114 tests passed.
• mise x -- pnpm run check — Svelte check passed with 0 errors and 0 warnings.
• mise x -- pnpm run lint and mise license-check passed.
• mise x -- pnpm exec playwright test e2e/user-settings.test.ts --retries=0 — all 8 browser tests passed.
• mise x -- pnpm run build passed; the en-US payload is approximately 14 KB gzipped.

Compatibility: existing en preferences migrate automatically; there are no protobuf, public API, security, or operational changes.