fix: USB badge detection for bind-mounted drives + graceful Tier2 backup on disconnected destinations

- IsUSBDevice/diskModel: strip findmnt bind-mount suffix [/subdir] before
  parsing device path (fixes USB badge not showing for attach-wizard drives)
- crossdrive.go: skip disconnected src/dest drives with WARN log instead of
  returning error (prevents noisy error status in settings.json)
- handlers.go: detect Tier2 destination disconnection, set yellow status dot
  instead of red, skip ValidateDestination for disconnected paths
- backups.html: new template branch showing "Cél meghajtó leválasztva" badge
  with grayed-out info and hidden "Futtatás most" button

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

controller/internal/system/mounts_linux.go

@@ -248,6 +248,10 @@ func stripPartition(base string) string {
 
 // diskModel reads the disk model from /sys/block/<dev>/device/model.
 func diskModel(device string) string {
+	// Strip bind-mount subdir suffix (e.g., "/dev/sdb1[/felhom_data]" → "/dev/sdb1")
+	if idx := strings.IndexByte(device, '['); idx >= 0 {
+		device = device[:idx]
+	}
 	disk := stripPartition(filepath.Base(device))
 	modelPath := "/sys/block/" + disk + "/device/model"
 	data, err := os.ReadFile(modelPath)
@@ -321,8 +325,13 @@ func ProbeStoragePath(path string) ProbeResult {
 
 // IsUSBDevice checks if a block device is connected via USB.
 // devicePath should be like "/dev/sdb" or "/dev/sdb1".
+// Also handles bind-mount suffixes from findmnt (e.g., "/dev/sdb1[/felhom_data]").
 // Checks the sysfs symlink for the disk — if the path contains "/usb", it's a USB device.
 func IsUSBDevice(devicePath string) bool {
+	// Strip bind-mount subdir suffix (e.g., "/dev/sdb1[/felhom_data]" → "/dev/sdb1")
+	if idx := strings.IndexByte(devicePath, '['); idx >= 0 {
+		devicePath = devicePath[:idx]
+	}
 	disk := stripPartition(filepath.Base(devicePath))
 	if disk == "" {
 		return false