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/components/Sidebar.tsx

@@ -6,6 +6,7 @@ import Link from 'next/link';
 import { useSelectedLayoutSegments } from 'next/navigation';
 import React, { useState, type ReactNode } from 'react';
 import Layout from './Layout';
+import { useTranslations } from 'next-intl';
 
 const VerticalIconContainer = ({ children }: { children: ReactNode }) => {
   return (
@@ -15,25 +16,26 @@ const VerticalIconContainer = ({ children }: { children: ReactNode }) => {
 
 const Sidebar = ({ children }: { children: React.ReactNode }) => {
   const segments = useSelectedLayoutSegments();
+  const t = useTranslations('navigation');
 
   const navLinks = [
     {
       icon: Home,
       href: '/',
       active: segments.length === 0 || segments.includes('c'),
-      label: 'Home',
+      label: t('home'),
     },
     {
       icon: Search,
       href: '/discover',
       active: segments.includes('discover'),
-      label: 'Discover',
+      label: t('discover'),
     },
     {
       icon: BookOpenText,
       href: '/library',
       active: segments.includes('library'),
-      label: 'Library',
+      label: t('library'),
     },
   ];