chore(i18n): translation contribution guardrails

[ParadoxInfinite]

Sets up community translations now that the UI is translatable (#118), with
guardrails so a broken catalog can't merge.

What's here

• web/scripts/check-i18n.mjs validates every catalog against the English source
  of truth: valid JSON, no key that English doesn't have, value shapes that match
  (string vs plural map), and a manifest that only lists locales with a catalog.
  Incomplete translations pass — missing keys fall back to English — and the
  script prints per-language coverage. Runnable as npm run check-i18n.
• CI runs the check on every PR, so a typo'd key or malformed plural fails before
  merge rather than silently showing English.
• A Translations section in CONTRIBUTING with the three steps to add a language.
• A Translation issue form (labeled help wanted / good first issue) to propose or
  claim a language.

Testing

• The check passes on the current catalogs and exits non-zero on an injected bad
  locale (unknown key, wrong value shape, missing manifest entry).