Add camera/webcam capture engine with engine-aware display picker

- New CameraEngine using OpenCV VideoCapture for webcam capture
- HAS_OWN_DISPLAYS class attribute on CaptureEngine base to distinguish
  engines with their own device lists from desktop monitor engines
- Display picker renders device list for cameras/scrcpy, spatial layout
  for desktop monitors
- Engine-aware display label formatting (camera name vs monitor index)
- Stream modal properly loads engine-specific displays on template change,
  edit, and clone
- Camera backend config rendered as dropdown (auto/dshow/msmf/v4l2)
- Remove offline label from device cards (healthcheck indicator suffices)

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

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

@@ -228,6 +228,7 @@ async def list_engines(_auth: AuthRequired):
                     name=engine_type.upper(),
                     default_config=engine_class.get_default_config(),
                     available=(engine_type in available_set),
+                    has_own_displays=getattr(engine_class, 'HAS_OWN_DISPLAYS', False),
                 )
             )