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/app/manifest.ts

@@ -1,11 +1,12 @@
 import type { MetadataRoute } from 'next';
+import { getTranslations } from 'next-intl/server';
 
-export default function manifest(): MetadataRoute.Manifest {
+export default async function manifest(): Promise<MetadataRoute.Manifest> {
+  const t = await getTranslations('manifest');
   return {
-    name: 'Perplexica - Chat with the internet',
-    short_name: 'Perplexica',
-    description:
-      'Perplexica is an AI powered chatbot that is connected to the internet.',
+    name: t('name'),
+    short_name: t('shortName'),
+    description: t('description'),
     start_url: '/',
     display: 'standalone',
     background_color: '#0a0a0a',