Fix Silas identity/voice leak, avatar gender, and add pre-warm batch gen

session.caller was reading name/voice from CALLER_BASES randomized defaults
instead of the slim bg dict — causing Silas to appear as "Earl" with wrong
voice. Now prefers bg data when populated.

Also: avatar endpoint infers gender from voice pool, /api/callers returns bg
name, regulars get canonical voice force-locked, batch gen splits into two
parallel calls with pre-warm so subsequent resets are instant.

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

frontend/index.html

@@ -324,6 +324,6 @@
         </div>
     </div>
 
-    <script src="/js/app.js?v=27"></script>
+    <script src="/js/app.js?v=28"></script>
 </body>
 </html>

frontend/js/app.js

@@ -744,18 +744,32 @@ async function newSession() {
         await hangup();
     }
 
-    await fetch('/api/session/reset', { method: 'POST' });
-    conversationSince = 0;
+    const btn = document.getElementById('new-session-btn');
+    const originalText = btn?.textContent;
+    if (btn) {
+        btn.disabled = true;
+        btn.textContent = 'Generating...';
+    }
 
-    // Hide caller background
-    const bgDetails = document.getElementById('caller-background-details');
-    if (bgDetails) bgDetails.classList.add('hidden');
+    try {
+        await fetch('/api/session/reset', { method: 'POST' });
+        conversationSince = 0;
 
-    // Reload callers to get new session ID
-    await loadCallers();
-    await loadShowTheme();
+        // Hide caller background
+        const bgDetails = document.getElementById('caller-background-details');
+        if (bgDetails) bgDetails.classList.add('hidden');
 
-    log('New session started - all callers have fresh backgrounds');
+        // Reload callers to get new session ID
+        await loadCallers();
+        await loadShowTheme();
+
+        log('New session started - all callers have fresh backgrounds');
+    } finally {
+        if (btn) {
+            btn.disabled = false;
+            btn.textContent = originalText || 'New Session';
+        }
+    }
 }