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

auto-imports.d.ts

@@ -6,5 +6,5 @@
 // biome-ignore lint: disable
 export {}
 declare global {
-
+  const showToast: typeof import('vant/es').showToast
 }