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/backup/crossdrive.go

@@ -90,25 +90,15 @@ func (r *CrossDriveRunner) RunAppBackup(ctx context.Context, stackName string) e
 		r.mu.Unlock()
 	}()
 
-	// Check if source or destination drive is disconnected
+	// Check if source or destination drive is disconnected — skip silently (not an error)
 	srcDrive := r.stackProvider.GetStackHDDPath(stackName)
 	if srcDrive != "" && r.sett.IsDisconnected(srcDrive) {
-		if r.debug {
-			r.logger.Printf("[DEBUG] RunAppBackup: source drive disconnected for %s: %s", stackName, srcDrive)
-		}
-		r.mu.Lock()
-		r.running[stackName] = false
-		r.mu.Unlock()
-		return fmt.Errorf("source drive disconnected: %s", srcDrive)
+		r.logger.Printf("[WARN] [backup] Cross-drive backup skipped for %s: source drive disconnected (%s)", stackName, srcDrive)
+		return nil
 	}
 	if r.sett.IsDisconnected(cfg.DestinationPath) {
-		if r.debug {
-			r.logger.Printf("[DEBUG] RunAppBackup: destination drive disconnected for %s: %s", stackName, cfg.DestinationPath)
-		}
-		r.mu.Lock()
-		r.running[stackName] = false
-		r.mu.Unlock()
-		return fmt.Errorf("destination drive disconnected: %s", cfg.DestinationPath)
+		r.logger.Printf("[WARN] [backup] Cross-drive backup skipped for %s: destination drive disconnected (%s)", stackName, cfg.DestinationPath)
+		return nil
 	}
 
 	// Mark as running in settings