feat: desktop GUI enhancements — audio level, call timer, VPIO, settings

- Audio level meter with log-scale RMS visualization
- Call duration timer
- VPIO (OS AEC) wired through to engine with fallback to CPAL
- "You" badge on own participant entry
- Recent rooms list (click to reuse)
- Enter key to connect from form fields
- Improved dark theme with pulse animation on status dot
- Settings persistence via localStorage (relay, room, alias, AEC, recent rooms)
- Fingerprint display on connect screen
- Keyboard shortcuts skip input fields

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/index.html

@@ -11,6 +11,7 @@
       <!-- Connect screen -->
       <div id="connect-screen">
         <h1>WarzonePhone</h1>
+        <p class="subtitle">Encrypted Voice</p>
         <div class="form">
           <label>Relay
             <input id="relay" type="text" value="193.180.213.68:4433" />
@@ -21,36 +22,47 @@
           <label>Alias
             <input id="alias" type="text" placeholder="your name" />
           </label>
-          <label class="checkbox">
-            <input id="os-aec" type="checkbox" checked />
-            OS Echo Cancellation
-          </label>
+          <div class="form-row">
+            <label class="checkbox">
+              <input id="os-aec" type="checkbox" checked />
+              OS Echo Cancel
+            </label>
+          </div>
           <button id="connect-btn" class="primary">Connect</button>
           <p id="connect-error" class="error"></p>
         </div>
+        <div class="identity-info">
+          <span id="my-fingerprint" class="fp-display"></span>
+        </div>
+        <div class="recent-rooms" id="recent-rooms"></div>
       </div>
 
       <!-- In-call screen -->
       <div id="call-screen" class="hidden">
         <div class="call-header">
           <div id="room-name" class="room-name"></div>
-          <div id="call-status" class="status">Connected</div>
+          <div class="call-meta">
+            <span id="call-status" class="status-dot"></span>
+            <span id="call-timer" class="call-timer">0:00</span>
+          </div>
+        </div>
+
+        <!-- Audio level meter -->
+        <div class="level-meter">
+          <div id="level-bar" class="level-bar-fill"></div>
         </div>
 
         <div id="participants" class="participants"></div>
 
         <div class="controls">
           <button id="mic-btn" class="control-btn" title="Toggle Mic (m)">
-            <span class="icon">🎙</span>
-            <span class="label">Mic</span>
-          </button>
-          <button id="spk-btn" class="control-btn" title="Toggle Speaker (s)">
-            <span class="icon">🔊</span>
-            <span class="label">Speaker</span>
+            <span class="icon" id="mic-icon">Mic</span>
           </button>
           <button id="hangup-btn" class="control-btn hangup" title="Hang Up (q)">
-            <span class="icon">📞</span>
-            <span class="label">End</span>
+            <span class="icon">End</span>
+          </button>
+          <button id="spk-btn" class="control-btn" title="Toggle Speaker (s)">
+            <span class="icon" id="spk-icon">Spk</span>
           </button>
         </div>