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/style.css

@@ -579,3 +579,69 @@ button:hover {
     white-space: nowrap;
     min-width: 200px;
 }
+
+/* AI Sync panel (Phase 6) */
+.ai-sync-panel {
+    background: var(--bg-panel-dark);
+    border: 1px solid var(--border-dim);
+    margin: 0 4px;
+    padding: 0;
+    font-family: var(--font-mono);
+}
+.ai-sync-header {
+    background: var(--bg-panel);
+    padding: 4px 8px;
+    font-size: 11px;
+    color: var(--accent);
+    text-transform: uppercase;
+    letter-spacing: 1px;
+    border-bottom: 1px solid var(--border-dim);
+}
+.ai-sync-body {
+    padding: 6px 8px;
+    display: flex;
+    align-items: center;
+    gap: 12px;
+}
+.ai-sync-info {
+    font-size: 11px;
+    color: var(--text-dim);
+    min-width: 140px;
+}
+.ai-sync-toggles {
+    display: flex;
+    gap: 8px;
+}
+.ai-sync-toggle {
+    font-size: 11px;
+    color: var(--text-primary);
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    gap: 3px;
+}
+.ai-sync-toggle input[type="checkbox"] {
+    accent-color: var(--accent);
+}
+.ai-sync-actions {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    margin-left: auto;
+}
+.ai-sync-generate {
+    background: var(--accent);
+    color: var(--bg-primary);
+    font-weight: bold;
+    padding: 3px 12px;
+}
+.ai-sync-generate:disabled {
+    opacity: 0.3;
+    cursor: not-allowed;
+}
+.ai-sync-llm-toggle {
+    opacity: 0.4;
+}
+.ai-sync-llm-toggle:not(.disabled) {
+    opacity: 1;
+}