Add configurable FPS to test preview and fix composite stream release race

- Add FPS control (1-60, default 20) to test preview modal next to LED count
- Server accepts fps query param, controls frame send interval
- Single Apply icon button (✓) applies both LED count and FPS
- FPS control stays visible for picture sources (LED count hidden)
- Fix composite sub-stream consumer ID collision: use unique instance ID
  to prevent old WebSocket release from killing new connection's streams

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

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

@@ -682,6 +682,7 @@ async def test_color_strip_ws(
     source_id: str,
     token: str = Query(""),
     led_count: int = Query(100),
+    fps: int = Query(20),
 ):
     """WebSocket for real-time CSS source preview. Auth via ``?token=<api_key>``.
 
@@ -723,8 +724,12 @@ async def test_color_strip_ws(
     if hasattr(stream, "configure"):
         stream.configure(max(1, led_count))
 
+    # Clamp FPS to sane range
+    fps = max(1, min(60, fps))
+    _frame_interval = 1.0 / fps
+
     await websocket.accept()
-    logger.info(f"CSS test WebSocket connected for {source_id}")
+    logger.info(f"CSS test WebSocket connected for {source_id} (fps={fps})")
 
     try:
         from wled_controller.core.processing.composite_stream import CompositeColorStripStream
@@ -846,7 +851,7 @@ async def test_color_strip_ws(
                     except Exception as e:
                         logger.warning(f"JPEG frame preview error: {e}")
 
-            await asyncio.sleep(0.05)
+            await asyncio.sleep(_frame_interval)
     except WebSocketDisconnect:
         pass
     except Exception as e: