Add theatre-mode dialog for the frame header preview thumbnail

Clicking the small preview thumbnail now opens an enlarged version in
a dialog (fresh render, same as the old click-to-refresh), closable
via the X button or a backdrop click -- mirroring #widget-dialog's
existing pattern.

server/app/static/frame_header.js

@@ -60,17 +60,51 @@
 
 // Live "how it's displaying" thumbnail. A real composite render (same
 // pipeline /frame/image uses), not a cached snapshot, so it's on a slow
-// poll and also click-to-refresh rather than something tighter like the
-// 10s device-status poll -- no need to hit Immich/calendar/whiteboard
-// sources that often just for a header thumbnail.
+// poll rather than something tighter like the 10s device-status poll --
+// no need to hit Immich/calendar/whiteboard sources that often just for
+// a header thumbnail. Click enlarges it in a dialog (which also fetches
+// a fresh render); clicking the enlarged image refreshes it again.
 (function () {
   var thumb = document.getElementById('frame-preview-thumb');
+  var dialog = document.getElementById('frame-preview-dialog');
+  var bigImg = document.getElementById('frame-preview-dialog-img');
+  var closeBtn = document.getElementById('frame-preview-dialog-close');
   if (!thumb || !window.FRAME_BASE_API) return;
 
-  function refresh() {
-    thumb.src = `${window.FRAME_BASE_API}/preview?t=${Date.now()}`;
+  function previewUrl() {
+    return `${window.FRAME_BASE_API}/preview?t=${Date.now()}`;
+  }
+  function refreshThumb() {
+    thumb.src = previewUrl();
+  }
+  // Opening the dialog (or clicking the big image inside it) fetches a
+  // fresh render and keeps the header thumb in sync, so this single path
+  // covers both "enlarge" and the old click-to-refresh behavior.
+  function refreshBig() {
+    var url = previewUrl();
+    bigImg.src = url;
+    thumb.src = url;
+  }
+
+  thumb.addEventListener('click', function () {
+    if (!dialog) { refreshThumb(); return; }
+    refreshBig();
+    dialog.showModal();
+  });
+  refreshThumb();
+  setInterval(refreshThumb, 60000);
+
+  if (dialog && bigImg && closeBtn) {
+    bigImg.addEventListener('click', refreshBig);
+    closeBtn.addEventListener('click', function () { dialog.close(); });
+    // Same backdrop-click-to-close trick as #widget-dialog: a click that
+    // lands on the dialog element itself (not its content box) means the
+    // backdrop was hit.
+    dialog.addEventListener('click', function (e) {
+      if (e.target !== dialog) return;
+      var rect = dialog.getBoundingClientRect();
+      var inside = e.clientX >= rect.left && e.clientX <= rect.right && e.clientY >= rect.top && e.clientY <= rect.bottom;
+      if (!inside) dialog.close();
+    });
   }
-  thumb.addEventListener('click', refresh);
-  refresh();
-  setInterval(refresh, 60000);
 })();

server/app/static/theme.css

@@ -652,6 +652,36 @@ code {
 }
 .frame-preview-thumb:hover { opacity: 0.8; }
 
+.frame-preview-dialog {
+  position: fixed;
+  margin: auto;
+  padding: 0;
+  border: 1px solid var(--border);
+  border-radius: 14px;
+  background: var(--surface);
+  box-shadow: var(--shadow-hover);
+  line-height: 0; /* avoid a baseline gap under the image */
+}
+.frame-preview-dialog::backdrop { background: var(--overlay); }
+.frame-preview-dialog img {
+  display: block;
+  max-width: min(90vw, 900px);
+  max-height: min(85vh, 900px);
+  width: auto;
+  height: auto;
+  border-radius: 14px;
+  cursor: pointer;
+}
+#frame-preview-dialog-close {
+  position: absolute;
+  top: 14px;
+  right: 14px;
+  width: 30px;
+  height: 30px;
+  font-size: 18px;
+  z-index: 1;
+}
+
 .control-banner {
   display: flex;
   align-items: center;

server/app/templates/_frame_name_edit.html

@@ -7,5 +7,10 @@
   <button type="button" class="btn-inline" id="frame-name-save">Save</button>
   <button type="button" class="btn-inline secondary" id="frame-name-cancel">Cancel</button>
 </span>
-<img id="frame-preview-thumb" class="frame-preview-thumb" alt="Live preview of what the frame is displaying" title="What the frame is currently displaying -- click to refresh">
+<img id="frame-preview-thumb" class="frame-preview-thumb" alt="Live preview of what the frame is displaying" title="What the frame is currently displaying -- click to enlarge">
+
+<dialog id="frame-preview-dialog" class="frame-preview-dialog">
+  <button type="button" id="frame-preview-dialog-close" class="icon-btn" aria-label="Close" title="Close">&times;</button>
+  <img id="frame-preview-dialog-img" alt="Live preview of what the frame is displaying" title="Click to refresh">
+</dialog>