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/app/photo_queue.py

@@ -20,7 +20,7 @@ import time
 
 from .config import FrameConfig
 
-QUEUE_TARGET_LEN = 10
+QUEUE_TARGET_LEN = 24
 
 
 def _top_up(cfg: FrameConfig, assets: list[dict]) -> None: