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/templates/modals/stream.html

@@ -24,7 +24,7 @@
                         </div>
                         <small class="input-hint" style="display:none" data-i18n="streams.display.hint">Which screen to capture</small>
                         <input type="hidden" id="stream-display-index" value="">
-                        <button type="button" class="btn btn-display-picker" id="stream-display-picker-btn" onclick="openDisplayPicker(onStreamDisplaySelected, document.getElementById('stream-display-index').value, document.getElementById('stream-capture-template').selectedOptions[0]?.dataset?.engineType)">
+                        <button type="button" class="btn btn-display-picker" id="stream-display-picker-btn" onclick="openDisplayPicker(onStreamDisplaySelected, document.getElementById('stream-display-index').value, document.getElementById('stream-capture-template').selectedOptions[0]?.dataset?.hasOwnDisplays === '1' ? document.getElementById('stream-capture-template').selectedOptions[0].dataset.engineType : null)">
                             <span id="stream-display-picker-label" data-i18n="displays.picker.select">Select display...</span>
                         </button>
                     </div>