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/templates/_widget_dialog_photos.html

@@ -46,6 +46,10 @@
 <section class="card" style="margin-top: 20px;">
   <h2 class="card-title">Now displaying</h2>
   <div id="current-thumb"><p class="sub">Loading...</p></div>
+  <button type="button" class="secondary" id="lock-photo-btn" style="margin-top: 8px;">Lock this photo</button>
+  <p class="sub" style="margin-top: 4px;">While locked, this photo stays
+    on screen -- the refresh timer and the next/back buttons won't
+    change it until you unlock it.</p>
 </section>
 
 <section class="card" style="margin-top: 20px;">