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

src-tauri/tauri.conf.json

@@ -20,7 +20,9 @@
         "minHeight": 600,
         "center": true,
         "resizable": true,
-        "fullscreen": false
+        "fullscreen": false,
+        "decorations": false,
+        "transparent": true
       }
     ],
     "security": {
@@ -28,7 +30,7 @@
     }
   },
   "bundle": {
-    "active": true,
+    "active": false,
     "targets": "all",
     "icon": [
       "icons/32x32.png",