feat(02-03): redesign frontend — browser audio element, beat markers, animation selector

- index.html: add <audio id=player> (hidden), beat-indicator dot in header
- index.html: animation selector panel with Beat Pulse/Strobe/Color Wave/Rainbow options
- index.html: remove seek slider + time display (browser audio element has native controls)
- index.html: upload button + file selector remain in transport bar
- app.js: wire <audio> element — play/pause/seeked/timeupdate events send WS ticks
- app.js: 10Hz setInterval tick loop (supplements native timeupdate ~4Hz)
- app.js: WS beat message -> flashBeatIndicator() animation
- app.js: loadWaveformAndBeats() draws waveform peaks + beat marker lines on canvas
- app.js: loadSelectedFile() sets audio.src=/shows/filename, fetches beats+waveform
- app.js: upload response caches beat data returned from server immediately
- style.css: .beat-indicator with .beat-flash (yellow glow animation)
- style.css: .animation-select-input for the animations panel dropdown

lightsync/frontend/style.css

@@ -337,3 +337,35 @@ button:hover {
     transition: left 0.1s linear;
     z-index: 10;
 }
+
+/* Beat indicator in header */
+.beat-indicator {
+    width: 10px;
+    height: 10px;
+    border-radius: 50%;
+    background: var(--text-dim);
+    flex-shrink: 0;
+    transition: background 0.05s ease;
+}
+
+.beat-indicator.beat-flash {
+    background: #ffcc00;
+    box-shadow: 0 0 8px #ffcc00;
+}
+
+/* Animation selector */
+.animation-select-input {
+    background: var(--bg-primary);
+    color: var(--text-primary);
+    border: 1px solid var(--border-dim);
+    padding: 4px 8px;
+    font-family: var(--font-mono);
+    font-size: var(--font-size);
+    width: 100%;
+    cursor: pointer;
+}
+
+.animation-select-input:focus {
+    border-color: var(--border-bright);
+    outline: none;
+}