Fix stale frame in API CSS preview when source is inactive

Server: send initial frame immediately after metadata for api_input
sources so the client gets the current buffer (fallback color if
inactive) instead of never receiving a frame when push_generation
stays at 0.

Frontend: clear all test modal canvases on open to prevent stale
pixels from previous sessions. Reset FPS timestamps after metadata
so the initial bootstrap frame isn't counted in the FPS chart.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -890,6 +890,14 @@ async def test_color_strip_ws(
             meta["layer_infos"] = layer_infos
         await websocket.send_text(_json.dumps(meta))
 
+        # For api_input: send the current buffer immediately so the client
+        # gets a frame right away (fallback color if inactive) rather than
+        # leaving the canvas blank/stale until external data arrives.
+        if is_api_input:
+            initial_colors = stream.get_latest_colors()
+            if initial_colors is not None:
+                await websocket.send_bytes(initial_colors.tobytes())
+
         # For picture sources, grab the live stream for frame preview
         _frame_live = None
         if is_picture and hasattr(stream, 'live_stream'):