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/api/routes/output_targets.py

@@ -98,7 +98,7 @@ def _target_to_response(target) -> OutputTargetResponse:
             target_type=target.target_type,
             device_id=target.device_id,
             color_strip_source_id=target.color_strip_source_id,
-            brightness_value_source_id=target.brightness_value_source_id,
+            brightness_value_source_id=target.brightness_value_source_id or "",
             fps=target.fps,
             keepalive_interval=target.keepalive_interval,
             state_check_interval=target.state_check_interval,