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/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;