feat: add runtime i18n with English, Vietnamese, and Simplified Chinese support

- Implement runtime i18n using MutationObserver for automatic DOM translation
- Add language switcher dropdown in dashboard header (EN/VI/ZH)
- Support 3 languages: English (default), Tiếng Việt, 简体中文
- Add translation files: vi.json (197 entries), zh-CN.json (513 entries, cleaned)
- Translate dashboard UI: sidebar menu, header, settings, MITM page
- Use cookie-based locale persistence with /api/locale endpoint
- Zero component changes required - translations applied at runtime
- Fix Header flicker on route change with key={pathname}

Co-authored-by: eachann <each1024@qq.com>
Based on PR #247 from decolua/9router with runtime approach

Made-with: Cursor

src/i18n/config.js

@@ -0,0 +1,26 @@
+export const LOCALES = ["en", "vi", "zh-CN"];
+export const DEFAULT_LOCALE = "en";
+export const LOCALE_COOKIE = "locale";
+
+export const LOCALE_NAMES = {
+  "en": "English",
+  "vi": "Tiếng Việt",
+  "zh-CN": "简体中文"
+};
+
+export function normalizeLocale(locale) {
+  if (locale === "zh" || locale === "zh-CN") {
+    return "zh-CN";
+  }
+  if (locale === "en") {
+    return "en";
+  }
+  if (locale === "vi") {
+    return "vi";
+  }
+  return DEFAULT_LOCALE;
+}
+
+export function isSupportedLocale(locale) {
+  return LOCALES.includes(locale);
+}