feat: expand color strip sources with gradient references and effect improvements

Add gradient_id field to color strip sources for referencing reusable
gradient entities. Improve audio stream processing and effect stream
with new parameters.

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

@@ -567,6 +567,13 @@ async def preview_color_strip_ws(
         if not stream_cls:
             raise ValueError(f"Unsupported preview source_type: {source.source_type}")
         s = stream_cls(source)
+        # Inject gradient store for palette resolution
+        if hasattr(s, "set_gradient_store"):
+            try:
+                from wled_controller.api.dependencies import get_gradient_store
+                s.set_gradient_store(get_gradient_store())
+            except Exception:
+                pass
         if hasattr(s, "configure"):
             s.configure(led_count)
         # Inject sync clock if requested