fix: replace vue3-flag-icons with custom CSS-only flag icons

- Remove vue3-flag-icons dependency (was causing slow loading)
- Create custom flag-icons.css with only cn and us flags (inline SVG)
- Update language-change.vue to use CSS-based flag icons
- Delete vue3-flag-icons type declaration file
- Flag icons now load instantly without any JavaScript overhead

frontend/package.json

@@ -65,7 +65,6 @@
     "vue-input-otp": "^0.3.2",
     "vue-router": "^5.0.3",
     "vue-sonner": "^2.0.9",
-    "vue3-flag-icons": "^0.1.4",
     "zod": "^4.3.6"
   },
   "devDependencies": {