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/test-template.html

@@ -9,7 +9,7 @@
             <div class="form-group">
                 <label data-i18n="templates.test.display">Display:</label>
                 <input type="hidden" id="test-template-display" value="">
-                <button type="button" class="btn btn-display-picker" id="test-display-picker-btn" onclick="openDisplayPicker(onTestDisplaySelected, document.getElementById('test-template-display').value, window.currentTestingTemplate?.engine_type)">
+                <button type="button" class="btn btn-display-picker" id="test-display-picker-btn" onclick="openDisplayPicker(onTestDisplaySelected, document.getElementById('test-template-display').value, window._engineHasOwnDisplays?.(window.currentTestingTemplate?.engine_type) ? window.currentTestingTemplate.engine_type : null)">
                     <span id="test-display-picker-label" data-i18n="displays.picker.select">Select display...</span>
                 </button>
             </div>