feat(dashboard): add i18n runtime with per-locale code-splitting (U3)

Initialize the browser i18next instance with lazy per-locale catalog loading,
localStorage->navigator->htmlTag detection, script-aware zh fallback, and the
shared @fusion/i18n config. Catalogs are synced from @fusion/i18n into a
gitignored app/locales/ (predev/prebuild) and imported app-relative so Vite
emits one chunk per locale/namespace (verified: 15 chunks, none inlined into
the main bundle). First paint is gated on i18nReady to avoid raw-key flashes;
<I18nextProvider> wraps the App provider stack; vendor-i18n manualChunk added.

A build-assertion script (verify:locale-chunks) guards the KTD3a splitting
invariant. Fallback/namespace behavior is covered by @fusion/i18n config tests;
the live instance is verified via the build assertion rather than a unit test
(the dynamic catalog backend is impractical to exercise in vitest).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/dashboard/package.json

@@ -52,8 +52,12 @@
     "README.md"
   ],
   "scripts": {
+    "gen:locales": "node scripts/sync-locales.mjs",
+    "verify:locale-chunks": "node scripts/assert-locale-chunks.mjs",
+    "prebuild": "node scripts/sync-locales.mjs",
     "build": "vite build && tsc",
     "build:client": "vite build",
+    "predev:serve": "node scripts/sync-locales.mjs",
     "dev": "pnpm build && pnpm typecheck && pnpm dev:serve",
     "dev:serve": "vite dev",
     "pretest": "node ../../scripts/ensure-test-artifacts.mjs",