feat(i18n): translatable UI with on-demand language catalogs

[ParadoxInfinite]

Makes the interface translatable, with languages delivered on demand so the
binary stays English-only and new translations ship without a release.

The translation layer

• t(key, params) and tn(key, count, params) resolve UI text against the active
  locale, with {placeholder} interpolation and Intl-based plural rules.
• English is bundled and is the per-key fallback, so a catalog that translates
  only some keys shows English for the rest.
• The chosen locale is saved to the browser (like the theme), detected from the
  saved choice, then the browser language, then English.

On-demand catalogs

• GET /api/i18n and GET /api/i18n/{locale} proxy the catalog files from the
  repo over a CDN, so a translation can be published or corrected without a new
  build. The browser talks only to the backend, which avoids CORS and works when
  outbound network is backend-only.
• Responses are size-capped, validated as JSON, and cached on disk for a few
  hours; a failed refresh still serves a stale cached copy. The locale tag is
  bounded so it can't reach another path or host.
• The UI pulls the published-locale list on startup and offers any it finds.

Coverage

• Every panel, dialog, component, and controller resolves its text through the
  layer. Label lists that were built once (table headers, nav, command palette,
  status pills) are reactive, so text updates on a language switch without reload.

Only English ships here; this is the machinery plus the English catalog. Strings
the backend produces (error text, raw engine states) are still shown as received.

Testing

• Backend tests cover the proxy cache, stale-on-outage fallback, non-JSON
  rejection, and tag validation.
• A check confirms every key referenced in the source exists in the catalog.
• npm run build, go build ./..., and go test ./... pass.