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/utils.ts

@@ -3,6 +3,22 @@ import { twMerge } from 'tailwind-merge';
 
 export const cn = (...classes: ClassValue[]) => twMerge(clsx(...classes));
 
+// Locale-aware absolute date formatting
+export const formatDate = (
+  date: Date | string,
+  locale: string,
+  options: Intl.DateTimeFormatOptions = {
+    year: 'numeric',
+    month: 'short',
+    day: '2-digit',
+    hour: '2-digit',
+    minute: '2-digit',
+  },
+) => {
+  const d = new Date(date);
+  return new Intl.DateTimeFormat(locale || undefined, options).format(d);
+};
+
 export const formatTimeDifference = (
   date1: Date | string,
   date2: Date | string,
@@ -25,3 +41,45 @@ export const formatTimeDifference = (
   else
     return `${Math.floor(diffInSeconds / 31536000)} year${Math.floor(diffInSeconds / 31536000) !== 1 ? 's' : ''}`;
 };
+
+// Locale-aware relative time using Intl.RelativeTimeFormat
+export const formatRelativeTime = (
+  date1: Date | string,
+  date2: Date | string,
+  locale: string,
+): string => {
+  const d1 = new Date(date1);
+  const d2 = new Date(date2);
+  const diffSeconds = Math.floor((d2.getTime() - d1.getTime()) / 1000); // positive if d2 > d1
+
+  const abs = Math.abs(diffSeconds);
+  let value: number;
+  let unit: Intl.RelativeTimeFormatUnit;
+
+  if (abs < 60) {
+    value = Math.round(diffSeconds);
+    unit = 'second';
+  } else if (abs < 3600) {
+    value = Math.round(diffSeconds / 60);
+    unit = 'minute';
+  } else if (abs < 86400) {
+    value = Math.round(diffSeconds / 3600);
+    unit = 'hour';
+  } else if (abs < 2629800) {
+    // ~1 month (30.4 days)
+    value = Math.round(diffSeconds / 86400);
+    unit = 'day';
+  } else if (abs < 31557600) {
+    // ~1 year
+    value = Math.round(diffSeconds / 2629800);
+    unit = 'month';
+  } else {
+    value = Math.round(diffSeconds / 31557600);
+    unit = 'year';
+  }
+
+  const rtf = new Intl.RelativeTimeFormat(locale || undefined, {
+    numeric: 'auto',
+  });
+  return rtf.format(value, unit);
+};