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/css/modal.css

@@ -723,6 +723,36 @@
     background: rgba(76, 175, 80, 0.12) !important;
 }
 
+/* ── Device picker list (cameras, scrcpy) ─────────────────────── */
+
+.device-picker-list {
+    display: flex;
+    flex-direction: column;
+    gap: 0.5rem;
+}
+
+.device-picker-item {
+    position: relative;
+    display: flex;
+    align-items: center;
+    gap: 0.75rem;
+    padding: 0.75rem 1rem;
+    border: 2px solid var(--border-color);
+    border-radius: 8px;
+    background: linear-gradient(135deg, rgba(128, 128, 128, 0.08), rgba(128, 128, 128, 0.03));
+    cursor: pointer;
+}
+
+.device-picker-item .layout-index-label {
+    position: static;
+    flex-shrink: 0;
+}
+
+.device-picker-item .layout-display-label {
+    text-align: left;
+    padding: 0;
+}
+
 /* ── Gradient editor ────────────────────────────────────────────── */
 
 .effect-palette-preview {