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/capabilities/default.json

@@ -6,6 +6,17 @@
     "main"
   ],
   "permissions": [
-    "core:default"
+    "core:default",
+    "core:window:default",
+    "core:window:allow-close",
+    "core:window:allow-minimize",
+    "core:window:allow-toggle-maximize",
+    "core:window:allow-is-maximized",
+    "core:window:allow-start-dragging",
+    "store:default",
+    "dialog:default",
+    "fs:default",
+    "fs:allow-read-dir",
+    "fs:allow-stat"
   ]
-}
+}