Add "now displaying" / "up next" preview pair to the frame header

The server now records exactly what was last sent to the device on
every device-facing render (/frame/image, /frame/advance, /frame/back,
and the global hold actions), persisted as Frame.last_displayed_image/
_at and served back via GET /api/frames/{id}/now-displaying. The
header thumbnail is split into that frozen "now displaying" snapshot
and the existing live "up next" re-render, with an arrow between them
-- so editing a layout shows the change immediately on the right while
the left stays exactly what's actually on the panel until the device's
next real wake.

server/app/static/theme.css

@@ -719,13 +719,24 @@ code {
 }
 .frame-name-edit button { margin-top: 0; }
 
+.frame-preview-pair {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  margin-left: 12px;
+  vertical-align: middle;
+}
+.frame-preview-arrow {
+  color: var(--text-muted);
+  font-size: 16px;
+  line-height: 1;
+}
 .frame-preview-thumb {
   height: 44px;
   width: auto;
   max-width: 130px;
   object-fit: contain;
   vertical-align: middle;
-  margin-left: 12px;
   border: 1px solid var(--border);
   border-radius: 4px;
   background: var(--surface-alt);
@@ -733,6 +744,13 @@ code {
   transition: opacity .12s ease;
 }
 .frame-preview-thumb:hover { opacity: 0.8; }
+.frame-preview-thumb-empty {
+  opacity: 0.3;
+  cursor: default;
+  width: 60px;
+  font-size: 0; /* no src yet -- suppresses the browser's fallback alt-text render */
+}
+.frame-preview-thumb-empty:hover { opacity: 0.3; }
 
 .frame-preview-dialog {
   position: fixed;