feat: comprehensive UI/a11y/i18n fixes and pre-push quality test

- Fix ClientWrappers missing 'about' namespace (MISSING_MESSAGE error)
- Add system/light/dark theme toggle with prefers-color-scheme detection
- Rewrite 404 page with i18n, accessibility, and proper navigation
- Rewrite books page with Header/Footer, i18n, and semantic HTML
- Add i18n keys to About, Footer, and both locale files
- Fix dark mode contrast: text-stone-300/600 -> text-stone-400
- Replace raw hex bg-[#fdfcf8] with bg-stone-50 across all components
- Guard console.error in ChatWidget and manage/page behind NODE_ENV
- Add aria-label to admin login form
- Remove emoji from manage page password toggle
- Update stale dates in privacy-policy and legal-notice
- Fix ScrollFadeIn index->delay prop type error in books page
- Fix privacy-policy and legal-notice landmark structure
- Add pre-push-check.test.ts: 13-category static analysis
  (i18n parity, namespace coverage, key resolution, accessibility,
   email validation, hex colors, emojis, console guards, env docs, types)
- Add explicit i18n check step to CI workflow

.gitea/workflows/ci.yml

@@ -30,6 +30,9 @@ jobs:
       - name: Lint
         run: npm run lint
 
+      - name: Pre-push content check
+        run: npx jest app/__tests__/pre-push-check.test.ts --no-coverage --verbose
+
       - name: Test
         run: npm run test