Replace camera switch button with device list dropdown

The camera drawer now offers a select of available video input devices
instead of a cycle button. "使用摄像头" starts the chosen camera and
changing the dropdown switches the live stream; the list is populated
from enumerateDevices (real labels after permission is granted).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/webpage/index.html

@@ -116,14 +116,14 @@
                   >
                     使用摄像头
                   </button>
-                  <button
-                    id="camera-flip-btn"
-                    class="btn btn--ghost camera-drawer__source"
-                    type="button"
+                  <select
+                    id="camera-device-select"
+                    class="camera-drawer__select"
+                    aria-label="选择摄像头"
                     disabled
                   >
-                    切换摄像头
-                  </button>
+                    <option value="">默认摄像头</option>
+                  </select>
                   <label
                     class="btn btn--ghost camera-drawer__source camera-drawer__source--upload"
                   >