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/lib/search/metaSearchAgent.ts

@@ -18,6 +18,7 @@ import LineOutputParser from '../outputParsers/lineOutputParser';
 import { getDocumentsFromLinks } from '../utils/documents';
 import { Document } from 'langchain/document';
 import { searchSearxng } from '../searxng';
+import { getLocale } from 'next-intl/server';
 import path from 'node:path';
 import fs from 'node:fs';
 import computeSimilarity from '../utils/computeSimilarity';
@@ -205,8 +206,10 @@ class MetaSearchAgent implements MetaSearchAgentType {
         } else {
           question = question.replace(/<think>.*?<\/think>/g, '');
 
+          const currentLocale = await getLocale();
+          const baseLang = (currentLocale?.split('-')[0] || 'en') as string;
           const res = await searchSearxng(question, {
-            language: 'en',
+            language: baseLang,
             engines: this.config.activeEngines,
           });