fix: Durchsuchen-Button, Tray-Icon, Minimize statt Close, .cloud Handler

1. Durchsuchen-Button: dialog:allow-open Permission in capabilities
2. Tray-Icon: Nutzt das App-Icon (32x32.png) statt leer
3. Close = Minimize: Fenster wird versteckt statt App beendet,
   Doppelklick auf Tray-Icon oeffnet wieder
4. .cloud Datei-Handler:
   - fileAssociations in tauri.conf.json registriert .cloud Extension
   - NSIS-Installer registriert den Handler automatisch
   - Doppelklick auf .cloud -> App startet, laedt Datei runter,
     oeffnet mit Standard-App (Word/Excel/etc.)
   - Wenn App laeuft: Event wird emitted, Frontend verarbeitet es

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clients/desktop/src-tauri/src/lib.rs

@@ -402,13 +402,40 @@ pub fn run() {
             locked_files: Mutex::new(Vec::new()),
             sync_paths: Mutex::new(Vec::new()),
         })
+        .on_window_event(|window, event| {
+            // Close button = minimize to tray instead of quit
+            if let tauri::WindowEvent::CloseRequested { api, .. } = event {
+                api.prevent_close();
+                let _ = window.hide();
+            }
+        })
         .setup(|app| {
             let quit = MenuItem::with_id(app, "quit", "Beenden", true, None::<&str>)?;
             let show = MenuItem::with_id(app, "show", "Oeffnen", true, None::<&str>)?;
             let sync_now = MenuItem::with_id(app, "sync", "Jetzt synchronisieren", true, None::<&str>)?;
             let menu = Menu::with_items(app, &[&show, &sync_now, &quit])?;
 
+            // Use bundled icon for tray
+            let icon = app.default_window_icon().cloned()
+                .unwrap_or_else(|| tauri::image::Image::from_bytes(include_bytes!("../icons/32x32.png")).unwrap());
+
+            // Handle .cloud file opened via file association (double-click)
+            let args: Vec<String> = std::env::args().collect();
+            if args.len() > 1 {
+                let file_arg = &args[1];
+                if file_arg.ends_with(".cloud") {
+                    let cloud_path = file_arg.to_string();
+                    let app_handle = app.handle().clone();
+                    // Open the .cloud file after app is ready
+                    std::thread::spawn(move || {
+                        std::thread::sleep(Duration::from_secs(2));
+                        let _ = app_handle.emit("open-cloud-file", cloud_path);
+                    });
+                }
+            }
+
             TrayIconBuilder::new()
+                .icon(icon)
                 .tooltip("Mini-Cloud Sync")
                 .menu(&menu)
                 .on_menu_event(|app, event| {
@@ -424,6 +451,15 @@ pub fn run() {
                         _ => {}
                     }
                 })
+                .on_tray_icon_event(|tray, event| {
+                    // Double-click on tray icon = show window
+                    if let tauri::tray::TrayIconEvent::DoubleClick { .. } = event {
+                        if let Some(w) = tray.app_handle().get_webview_window("main") {
+                            let _ = w.show();
+                            let _ = w.set_focus();
+                        }
+                    }
+                })
                 .build(app)?;
 
             Ok(())