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/Cargo.toml

@@ -23,3 +23,6 @@ serde = { version = "1.0", features = ["derive"] }
 log = "0.4"
 tauri = { version = "2.11.2", features = [] }
 tauri-plugin-log = "2"
+tauri-plugin-store = "2"
+tauri-plugin-dialog = "2"
+tauri-plugin-fs = "2"