Add Database page with new connection feature

- Reorganize project structure: move frontend to web/ directory
- Add Database page with connection list (name, type, subtables, status, created, actions)
- Integrate Element Plus for UI components with dark theme support
- Add Quicksand font for rounded UI design
- Configure root package.json to run frontend from web/ directory

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/postcss.config.js

@@ -0,0 +1,6 @@
+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+}