Add per-photo-widget lock (freezes current photo until unlocked)

A "Lock this photo" button in the photos widget's dialog toggles
PhotoWidgetConfig.locked, which suppresses both the timer-elapsed
auto-advance and the advance/back button actions until unlocked. The
Layout tab canvas shows a lock badge on any locked photo widget's box.

server/app/widgets/photos.py

@@ -55,7 +55,7 @@ def render(db: Session, frame: Frame, widget: Widget, target_w: int, target_h: i
 
 def _advance(db: Session, frame: Frame, widget: Widget) -> None:
     cfg = db.get(PhotoWidgetConfig, widget.id)
-    if not cfg.album_id:
+    if not cfg.album_id or cfg.locked:
         return
     try:
         client = immich_client_for(frame)
@@ -68,7 +68,7 @@ def _advance(db: Session, frame: Frame, widget: Widget) -> None:
 
 def _back(db: Session, frame: Frame, widget: Widget) -> None:
     cfg = db.get(PhotoWidgetConfig, widget.id)
-    if not cfg.album_id:
+    if not cfg.album_id or cfg.locked:
         return
     try:
         client = immich_client_for(frame)