feat(i18n): Integrate next-intl, localize core UI, add regional locales and zh-TW Discover sources

**Overview**
- Integrates next-intl (App Router, no i18n routing) with cookie-based locale and Accept-Language fallback.
- Adds message bundles and regional variants; sets en-US as the default.

**Key changes**
- i18n foundation
  - Adds request-scoped config to load messages per locale and injects NextIntlClientProvider in [layout.tsx]
  - Adds/updates messages for: en-US, en-GB, zh-TW, zh-HK, zh-CN, ja, ko, fr-FR, fr-CA, de.
Centralizes LOCALES, LOCALE_LABELS, and DEFAULT_LOCALE in [locales.ts]
  - Adds LocaleSwitcher (cookie-based) and [LocaleBootstrap]

- Pages and components
  - Localizes Sidebar, Home (including metadata/manifest), Settings, Discover, Library.
  - Localizes common components: MessageInput, Attach, Focus, Optimization, MessageBox, MessageSources, SearchImages, SearchVideos, EmptyChat, NewsArticleWidget, WeatherWidget.

- APIs
  - Weather API returns localized condition strings server-side.

- UX and quality
  - Converts all remaining <img> to Next Image.
  - Updates browserslist/caniuse DB to silence warnings.
  - Security: Settings API Key inputs are now password fields and placeholders were removed.

package.json

@@ -38,6 +38,7 @@
     "mammoth": "^1.9.1",
     "markdown-to-jsx": "^7.7.2",
     "next": "^15.2.2",
+    "next-intl": "^4.3.4",
     "next-themes": "^0.3.0",
     "pdf-parse": "^1.1.1",
     "react": "^18",