Add show improvement features: crossfade, emotions, returning callers, transcripts, screening

- Music crossfade: smooth 3-second blend between tracks instead of hard stop/start
- Emotional detection: analyze host mood from recent messages so callers adapt tone
- AI caller summaries: generate call summaries with timestamps for show history
- Returning callers: persist regular callers across sessions with call history
- Session export: generate transcripts with speaker labels and chapter markers
- Caller screening: AI pre-screens phone callers to get name and topic while queued

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

frontend/index.html

@@ -13,6 +13,7 @@
             <div class="header-buttons">
                 <button id="on-air-btn" class="on-air-btn off">OFF AIR</button>
                 <button id="new-session-btn" class="new-session-btn">New Session</button>
+                <button id="export-session-btn">Export</button>
                 <button id="settings-btn">Settings</button>
             </div>
         </header>