Restore panel build in CI, add browser fallback for Open Admin Panel

- Re-add CGO panel builds with apt-get install of gcc/g++/mingw-w64/webkit2gtk
- Panel launch falls back to default browser if panel binary not found
- Restore panel binary in nfpm .deb package

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/tray/panel.go

@@ -14,14 +14,14 @@ var (
 	panelProc *os.Process
 )
 
-// launchPanel starts the syncwarden-panel subprocess if not already running.
+// launchPanel opens the Syncthing admin UI.
+// Tries the embedded panel binary first, falls back to the default browser.
 func launchPanel(baseURL string) {
 	panelMu.Lock()
 	defer panelMu.Unlock()
 
-	// Check if already running
+	// Check if panel subprocess is already running
 	if panelProc != nil {
-		// Check if process is still alive
 		if err := panelProc.Signal(os.Signal(nil)); err == nil {
 			log.Println("panel already running")
 			return
@@ -29,33 +29,32 @@ func launchPanel(baseURL string) {
 		panelProc = nil
 	}
 
+	// Try embedded panel binary
 	panelBin := panelBinaryPath()
-	if panelBin == "" {
-		log.Println("syncwarden-panel binary not found")
-		return
+	if panelBin != "" {
+		cmd := exec.Command(panelBin, "-addr", baseURL)
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		if err := cmd.Start(); err == nil {
+			panelProc = cmd.Process
+			log.Printf("panel started (pid %d)", cmd.Process.Pid)
+			go func() {
+				_ = cmd.Wait()
+				panelMu.Lock()
+				panelProc = nil
+				panelMu.Unlock()
+				log.Println("panel exited")
+			}()
+			return
+		}
+		log.Printf("failed to start panel binary: %v", panelBin)
 	}
 
-	cmd := exec.Command(panelBin, "-addr", baseURL)
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-	if err := cmd.Start(); err != nil {
-		log.Printf("failed to start panel: %v", err)
-		return
-	}
-	panelProc = cmd.Process
-	log.Printf("panel started (pid %d)", cmd.Process.Pid)
-
-	// Wait for process to exit in background
-	go func() {
-		_ = cmd.Wait()
-		panelMu.Lock()
-		panelProc = nil
-		panelMu.Unlock()
-		log.Println("panel exited")
-	}()
+	// Fallback: open in default browser
+	log.Printf("opening %s in browser", baseURL)
+	openBrowser(baseURL)
 }
 
-// panelBinaryPath finds the syncwarden-panel binary next to the main binary.
 func panelBinaryPath() string {
 	exe, err := os.Executable()
 	if err != nil {
@@ -74,3 +73,18 @@ func panelBinaryPath() string {
 	}
 	return ""
 }
+
+func openBrowser(url string) {
+	var cmd *exec.Cmd
+	switch runtime.GOOS {
+	case "windows":
+		cmd = exec.Command("rundll32", "url.dll,FileProtocolHandler", url)
+	case "darwin":
+		cmd = exec.Command("open", url)
+	default:
+		cmd = exec.Command("xdg-open", url)
+	}
+	if err := cmd.Start(); err != nil {
+		log.Printf("failed to open browser: %v", err)
+	}
+}