Web UI: grid layout, drag-and-drop reorder, and "show next" for upcoming photos

Replaces the up/down-button vertical list with a responsive photo grid
(native HTML5 drag-and-drop between cards, reusing the existing
POST /api/queue/reorder endpoint -- no new server route needed). Each
card also gets a "Show next" button that jumps it straight to the front
of the queue.

Also bumps the queue lookahead from 10 to 24 photos (QUEUE_TARGET_LEN in
photo_queue.py) now that the grid has room to show more at once.

server/README.md

@@ -64,11 +64,12 @@ algorithm itself -- it just streams the response straight to the panel.
   mount. Immich URL/API key are too if set via the web UI, but
   `IMMICH_URL`/`IMMICH_API_KEY` env vars (see Setup above) always take
   precedence when present.
-- The upcoming queue is a bounded lookahead (10 photos), not the whole
-  album -- it's topped up automatically as photos are consumed
-  (`app/photo_queue.py`), in sequential or shuffle order per the Order
-  setting. Reordering only rearranges those 10; it doesn't add or remove
-  photos from the album.
+- The upcoming queue is a bounded lookahead (`QUEUE_TARGET_LEN` in
+  `app/photo_queue.py`, currently 24 photos), not the whole album -- it's
+  topped up automatically as photos are consumed, in sequential or
+  shuffle order per the Order setting. Dragging photos in the web UI (or
+  using "Show next") only rearranges what's already in that lookahead;
+  it doesn't add or remove photos from the album.
 - `/frame/image` and `/frame/advance` aren't authenticated yet. That's
   fine on a trusted home LAN for now, but worth revisiting once the ESP32
   side is wired up to send a shared device token.