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/core/processing/composite_stream.py

@@ -30,7 +30,9 @@ class CompositeColorStripStream(ColorStripStream):
     """
 
     def __init__(self, source, css_manager, value_stream_manager=None):
+        import uuid as _uuid
         self._source_id: str = source.id
+        self._instance_id: str = _uuid.uuid4().hex[:8]  # unique per instance to avoid release races
         self._layers: List[dict] = list(source.layers)
         self._led_count: int = source.led_count
         self._auto_size: bool = source.led_count == 0
@@ -167,7 +169,7 @@ class CompositeColorStripStream(ColorStripStream):
             src_id = layer.get("source_id", "")
             if not src_id:
                 continue
-            consumer_id = f"{self._source_id}__layer_{i}"
+            consumer_id = f"{self._source_id}__{self._instance_id}__layer_{i}"
             try:
                 stream = self._css_manager.acquire(src_id, consumer_id)
                 if hasattr(stream, "configure") and self._led_count > 0: