Fix device provider kwargs, camera crash guard, target API, and graph color picker

- Refactor all device providers to use explicit kwargs.get() instead of
  fragile pop-then-passthrough (fixes WLED target start failing with
  unexpected dmx_protocol kwarg)
- Add process-wide camera index registry to prevent concurrent opens of
  the same physical camera which crashes the DSHOW backend on Windows
- Fix OutputTargetResponse validation error when brightness_value_source_id
  is None (coerce to empty string in response and from_dict)
- Replace native <input type="color"> in graph editor with the custom
  color picker popover used throughout the app, positioned via
  getScreenCTM() inside an absolute overlay on .graph-container

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

server/src/wled_controller/storage/wled_output_target.py

@@ -114,7 +114,7 @@ class WledOutputTarget(OutputTarget):
             target_type="led",
             device_id=data.get("device_id", ""),
             color_strip_source_id=data.get("color_strip_source_id", ""),
-            brightness_value_source_id=data.get("brightness_value_source_id", ""),
+            brightness_value_source_id=data.get("brightness_value_source_id") or "",
             fps=data.get("fps", 30),
             keepalive_interval=data.get("keepalive_interval", 1.0),
             state_check_interval=data.get("state_check_interval", DEFAULT_STATE_CHECK_INTERVAL),