Add live LED strip preview via WebSocket on target cards

Stream real-time LED colors from running WLED targets to the browser via
binary WebSocket (RGB bytes, throttled to ~15 fps). Toggle button on
target card opens a compact canvas strip that renders each frame using
ImageData. Cached last frame is re-rendered after card reconciliation to
prevent flicker during auto-refresh.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/routes/picture_targets.py

@@ -693,6 +693,44 @@ async def target_colors_ws(
         manager.remove_kc_ws_client(target_id, websocket)
 
 
+@router.websocket("/api/v1/picture-targets/{target_id}/led-preview/ws")
+async def led_preview_ws(
+    websocket: WebSocket,
+    target_id: str,
+    token: str = Query(""),
+):
+    """WebSocket for real-time LED strip preview. Sends binary RGB frames. Auth via ?token=<api_key>."""
+    authenticated = False
+    cfg = get_config()
+    if token and cfg.auth.api_keys:
+        for _label, api_key in cfg.auth.api_keys.items():
+            if secrets.compare_digest(token, api_key):
+                authenticated = True
+                break
+
+    if not authenticated:
+        await websocket.close(code=4001, reason="Unauthorized")
+        return
+
+    await websocket.accept()
+
+    manager = get_processor_manager()
+
+    try:
+        manager.add_led_preview_client(target_id, websocket)
+    except ValueError:
+        await websocket.close(code=4004, reason="Target not found")
+        return
+
+    try:
+        while True:
+            await websocket.receive_text()
+    except WebSocketDisconnect:
+        pass
+    finally:
+        manager.remove_led_preview_client(target_id, websocket)
+
+
 # ===== STATE CHANGE EVENT STREAM =====