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/static/theme.css

@@ -477,6 +477,20 @@ button.secondary:hover { background: var(--surface-alt); }
 .widget-box-remove { right: 4px; }
 .widget-box-settings { right: 28px; }
 .widget-box-remove:hover, .widget-box-settings:hover { background: var(--overlay-hover); }
+.widget-box-lock-badge {
+  position: absolute;
+  bottom: 4px;
+  left: 4px;
+  width: 20px;
+  height: 20px;
+  border-radius: 50%;
+  background: var(--overlay);
+  color: #fff;
+  font-size: 11px;
+  line-height: 20px;
+  text-align: center;
+  pointer-events: none; /* passive indicator, not a control -- toggled from the widget's own dialog */
+}
 .widget-box-resize-handle {
   position: absolute;
   bottom: 0;