refactor: migrate login and agree popup to Vant and Tailwind

- replace uni-popup/uni-icons with Vant van-popup/van-icon in AgreePopup
- switch markdown rendering to vue3-markdown-it from zero-markdown-div
- remove unused AgreePopup demo, styles, README and image assets
- refactor login page to use Tailwind utilities instead of legacy SCSS
- replace uni.showToast with auto-imported vant showToast, add global type definition
- update package dependencies and refresh yarn.lock
- clean up unused uni-app specific imports and code

package.json

@@ -17,7 +17,8 @@
     "vant": "^4.9.24",
     "vue": "^3.5.13",
     "vue-i18n": "11.4.4",
-    "vue-router": "^4.5.1"
+    "vue-router": "^4.5.1",
+    "vue3-markdown-it": "^1.0.10"
   },
   "devDependencies": {
     "@tailwindcss/vite": "^4.1.12",