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/static/js/features/devices.js

@@ -73,7 +73,7 @@ export function createDeviceCard(device) {
         healthClass = 'health-offline';
         healthTitle = t('device.health.offline');
         if (state.device_error) healthTitle += `: ${state.device_error}`;
-        healthLabel = `<span class="health-latency offline">${t('device.health.offline')}</span>`;
+        healthLabel = '';
     }
 
     const ledCount = state.device_led_count || device.led_count;