feat(i18n): add runtime-switchable language support

[srsholmes]

Introduce an internationalization mechanism for the overlay app and its
plugins, with runtime language switching and per-plugin translation files.

Core (shared singleton in @loadout/ui):

• Add i18next + react-i18next; new packages/ui/src/i18n.ts hosts one
  i18next instance. Because plugin bundles resolve @loadout/ui to the
  shell's __LOADOUT_SDK global, the shell and every plugin share that
  instance — setLanguage() re-renders the whole tree with no reload.
• Expose initI18n, setLanguage, ensureNamespace, normalizeLocale,
  usePluginTranslation, SUPPORTED_LANGUAGES (en-gb, zh-cn) and DEFAULT.

Detection / persistence / override:

• First-run detection: host OS locale (new getSystemLocale RPC) ->
  navigator.language -> English, normalized to a supported code and
  persisted under the language user-config key.
• Settings > General gains a Language selector that switches at runtime.

Translation files:

• Shell strings live under the app namespace in
  apps/loadout-overlay/src/overlay/i18n/.json (statically bundled).
• Plugins ship an i18n/.json folder (flat key/value, one file per
language, plugin id = namespace), served by the loader at
/plugins//i18n/.json and lazy-loaded on first use; missing
keys fall back to English.

Proof of mechanism (full English source + partial Chinese):

• Wire the Settings screen and the battery-tracker plugin to t() keys.
• This PR is the mechanism only; bulk translation is left to a follow-up.

Docs + tests:

• Document the convention in CLAUDE.md.
• Unit-test normalizeLocale; keep battery-tracker spec green by resolving
  its assertions against the shipped en-gb.json.

Co-Authored-By: Claude Opus 4.8 [email protected]
Claude-Session: https://claude.ai/code/session_01XEKUyS4sptP4hoLgKCDxxu