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.

src/i18n/locales.ts

@@ -0,0 +1,40 @@
+export const LOCALES = [
+  'en-US',
+  'en-GB',
+  'zh-TW',
+  'zh-HK',
+  'zh-CN',
+  'ja',
+  'ko',
+  'fr-FR',
+  'fr-CA',
+  'de',
+] as const;
+export type AppLocale = (typeof LOCALES)[number];
+
+// Default locale for fallbacks
+export const DEFAULT_LOCALE: AppLocale = 'en-US';
+
+// UI labels for language options
+export const LOCALE_LABELS: Record<AppLocale, string> = {
+  'en-US': 'English (US)',
+  'en-GB': 'English (UK)',
+  'zh-TW': '繁體中文',
+  'zh-HK': '繁體中文（香港）',
+  'zh-CN': '简体中文',
+  ja: '日本語',
+  ko: '한국어',
+  'fr-FR': 'Français (France)',
+  'fr-CA': 'Français (Canada)',
+  de: 'Deutsch',
+};
+
+// Human-readable language name for prompt prefix
+export function getPromptLanguageName(loc: string): string {
+  const l = (loc || '').toLowerCase();
+  const match = (
+    Object.keys(LOCALE_LABELS) as Array<keyof typeof LOCALE_LABELS>
+  ).find((k) => k.toLowerCase() === l);
+  if (match) return LOCALE_LABELS[match];
+  return LOCALE_LABELS[DEFAULT_LOCALE];
+}