feat(ui): add Join Video button — joins call and auto-starts camera

Blue FAB alongside Join Voice; click handler connects then calls
startCamera() so video is active from the moment the call starts.
Cam button inside drawer still toggles camera after joining either way.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

desktop/index.html

@@ -43,12 +43,16 @@
           </div>
         </div>
 
-        <!-- Voice join FAB -->
+        <!-- Voice / Video join FABs -->
         <div class="lobby-fab-row">
           <button id="join-voice-btn" class="fab" title="Join Voice Chat">
             <span class="fab-icon">&#x1F3A7;</span>
             <span class="fab-label">Join Voice</span>
           </button>
+          <button id="join-video-btn" class="fab fab-video" title="Join with Video">
+            <span class="fab-icon">&#x1F4F9;</span>
+            <span class="fab-label">Join Video</span>
+          </button>
         </div>
 
         <!-- Incoming call banner -->