feat(06-03): add AI Sync panel UI and frontend wiring

- Add AI SYNC button to timeline toolbar, panel hidden by default (D-08)
- Panel: feature toggles (BEATS, ONSETS, CHROMA, RMS), GENERATE button, USE LLM toggle
- GENERATE disabled until audio loaded; re-enables on loadedmetadata event
- LLM toggle enabled/disabled based on /api/sync/llm-available at page load
- D-07: confirmation dialog before replacing existing timeline blocks
- D-11: section_range passed when a segment is selected in the overlay
- Segment selection watcher (200ms interval) updates info text dynamically
- Add AI sync panel styles — flat terminal aesthetic, no border-radius

lightsync/frontend/index.html

@@ -63,6 +63,27 @@
                 </label>
                 <div class="toolbar-sep"></div>
                 <button id="btn-snap" class="transport-btn snap-btn active" title="Snap to beat">SNAP</button>
+                <div class="toolbar-sep"></div>
+                <button id="btn-ai-sync-open" class="transport-btn" title="AI-assisted show generation">AI SYNC</button>
+            </div>
+            <div id="ai-sync-panel" class="ai-sync-panel" style="display:none;">
+                <div class="ai-sync-header">AI SYNC</div>
+                <div class="ai-sync-body">
+                    <div class="ai-sync-info" id="ai-sync-info">SELECT AUDIO FILE FIRST</div>
+                    <div class="ai-sync-toggles">
+                        <label class="ai-sync-toggle"><input type="checkbox" id="ai-use-beats" checked> BEATS</label>
+                        <label class="ai-sync-toggle"><input type="checkbox" id="ai-use-onsets" checked> ONSETS</label>
+                        <label class="ai-sync-toggle"><input type="checkbox" id="ai-use-chroma" checked> CHROMA</label>
+                        <label class="ai-sync-toggle"><input type="checkbox" id="ai-use-rms" checked> RMS</label>
+                    </div>
+                    <div class="ai-sync-actions">
+                        <button id="btn-ai-generate" class="transport-btn ai-sync-generate" disabled>GENERATE</button>
+                        <label class="ai-sync-toggle ai-sync-llm-toggle">
+                            <input type="checkbox" id="ai-use-llm" disabled>
+                            <span id="ai-llm-label">USE LLM</span>
+                        </label>
+                    </div>
+                </div>
             </div>
             <canvas id="timeline-canvas"></canvas>
             <div id="live-preview" class="live-preview" style="display:none;"></div>