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/app/api/locale/route.js

@@ -0,0 +1,30 @@
+import { cookies } from "next/headers";
+import { NextResponse } from "next/server";
+import { LOCALE_COOKIE, normalizeLocale, isSupportedLocale } from "@/i18n/config";
+
+export async function POST(request) {
+  try {
+    const { locale } = await request.json();
+    
+    if (!locale || !isSupportedLocale(locale)) {
+      return NextResponse.json(
+        { error: "Invalid locale" },
+        { status: 400 }
+      );
+    }
+
+    const normalized = normalizeLocale(locale);
+    const cookieStore = await cookies();
+    cookieStore.set(LOCALE_COOKIE, normalized, {
+      path: "/",
+      maxAge: 60 * 60 * 24 * 365, // 1 year
+    });
+
+    return NextResponse.json({ success: true, locale: normalized });
+  } catch (error) {
+    return NextResponse.json(
+      { error: "Failed to set locale" },
+      { status: 500 }
+    );
+  }
+}