Refactor: ES module architecture, tauri-plugin-store, unified design

Major changes:
- ES module system: main.js as central hub, shared modules
- tauri-plugin-store for persistent storage (dialog, fs plugins added)
- tauri-plugin-dialog for folder selection
- tauri-plugin-fs for real file system scanning
- Removed MOCK_FILES/FILE_TREE - real file scanner implemented
- All pages use unified page-header + page-content layout
- Custom titlebar with window controls
- Onboarding: 4-step flow with folder selection (Downloads default)
- CSS: unified design system, .hidden utility class added
- SVG icons throughout (no emoji)

Note: onboarding.js downloadDir() has runtime issue - needs fix

package.json

@@ -10,8 +10,13 @@
     "tauri": "tauri"
   },
   "dependencies": {
+    "@fontsource/noto-color-emoji": "^5.2.12",
     "@fontsource/noto-sans-kr": "^5.2.9",
-    "@fontsource/outfit": "^5.2.8"
+    "@fontsource/outfit": "^5.2.8",
+    "@tauri-apps/api": "^2.11.0",
+    "@tauri-apps/plugin-dialog": "^2.7.1",
+    "@tauri-apps/plugin-fs": "^2.5.1",
+    "@tauri-apps/plugin-store": "^2.4.3"
   },
   "devDependencies": {
     "@tauri-apps/cli": "^2.0.0",