feat(02-03): build transport UI and waveform canvas

- Replace timeline placeholder with waveform canvas + playback cursor
- Replace transport placeholder with full transport bar controls
- Add play/pause/stop buttons, seek slider, time display, audio path input
- Add drawWaveform(), loadWaveform(), updatePosition(), formatTime() to app.js
- Dispatch msg.type=tick to updatePosition in handleMessage
- Wire transport buttons via WebSocket commands
- Add transport/waveform styles to style.css using existing CSS variables
- Deploy via ~/bin/deploy.sh

lightsync/frontend/app.js

@@ -56,8 +56,12 @@ class LightSyncClient {
     }
 
     handleMessage(msg) {
-        // Phase 2 will dispatch on msg.type
-        console.debug("[ws]", msg);
+        if (msg.type === 'tick') {
+            updatePosition(msg.position, msg.duration, msg.paused);
+            audioLoaded = msg.loaded;
+        } else {
+            console.debug('[ws]', msg);
+        }
     }
 
     send(msg) {
@@ -67,6 +71,86 @@ class LightSyncClient {
     }
 }
 
+// --- Waveform rendering ---
+let waveformPeaks = [];
+let audioDuration = 0;
+let audioLoaded = false;
+
+function drawWaveform(canvas, peaks) {
+    const ctx = canvas.getContext('2d');
+    const dpr = window.devicePixelRatio || 1;
+    const rect = canvas.getBoundingClientRect();
+    canvas.width = rect.width * dpr;
+    canvas.height = rect.height * dpr;
+    ctx.scale(dpr, dpr);
+
+    const w = rect.width;
+    const h = rect.height;
+    const mid = h / 2;
+    const barWidth = w / peaks.length;
+
+    ctx.clearRect(0, 0, w, h);
+
+    // Draw waveform bars using --accent color
+    ctx.fillStyle = getComputedStyle(document.documentElement).getPropertyValue('--accent').trim() || '#0ff';
+    for (let i = 0; i < peaks.length; i++) {
+        const amp = peaks[i] * mid * 0.9;
+        const x = i * barWidth;
+        ctx.fillRect(x, mid - amp, Math.max(barWidth - 0.5, 0.5), amp * 2);
+    }
+}
+
+async function loadWaveform() {
+    try {
+        const res = await fetch('/api/audio/waveform?peaks=2000');
+        if (!res.ok) return;
+        const data = await res.json();
+        waveformPeaks = data.peaks;
+        const canvas = document.getElementById('waveform-canvas');
+        if (canvas) drawWaveform(canvas, waveformPeaks);
+    } catch (err) {
+        console.error('[waveform]', err);
+    }
+}
+
+// --- Time formatting ---
+function formatTime(seconds) {
+    if (seconds == null || isNaN(seconds)) return '0:00.0';
+    const m = Math.floor(seconds / 60);
+    const s = seconds % 60;
+    return `${m}:${s < 10 ? '0' : ''}${s.toFixed(1)}`;
+}
+
+// --- Position update from WebSocket tick ---
+function updatePosition(position, duration, paused) {
+    audioDuration = duration || 0;
+
+    document.getElementById('time-current').textContent = formatTime(position);
+    document.getElementById('time-duration').textContent = formatTime(duration);
+
+    // Update seek bar
+    const seekBar = document.getElementById('seek-bar');
+    if (duration > 0 && !seekBar._dragging) {
+        seekBar.value = (position / duration) * 100;
+    }
+
+    // Update playback cursor position
+    const cursor = document.getElementById('playback-cursor');
+    const container = document.querySelector('.waveform-container');
+    if (cursor && container && duration > 0) {
+        const pct = (position / duration) * 100;
+        cursor.style.left = pct + '%';
+    }
+
+    // Toggle play/pause button visibility
+    const btnPlay = document.getElementById('btn-play');
+    const btnPause = document.getElementById('btn-pause');
+    if (btnPlay && btnPause) {
+        btnPlay.style.display = paused ? '' : 'none';
+        btnPause.style.display = paused ? 'none' : '';
+    }
+}
+
 const client = new LightSyncClient();
 client.connect();
 
@@ -115,6 +199,71 @@ function escapeHtml(str) {
 
 loadDevices();
 
+// --- Transport controls ---
+document.getElementById('btn-play')?.addEventListener('click', () => {
+    client.send({ type: 'play' });
+});
+
+document.getElementById('btn-pause')?.addEventListener('click', () => {
+    client.send({ type: 'pause' });
+});
+
+document.getElementById('btn-stop')?.addEventListener('click', () => {
+    client.send({ type: 'pause' });
+    client.send({ type: 'seek', position: 0 });
+});
+
+// Seek bar interaction
+const seekBar = document.getElementById('seek-bar');
+if (seekBar) {
+    seekBar.addEventListener('mousedown', () => { seekBar._dragging = true; });
+    seekBar.addEventListener('mouseup', () => {
+        seekBar._dragging = false;
+        const pos = (parseFloat(seekBar.value) / 100) * audioDuration;
+        client.send({ type: 'seek', position: pos });
+    });
+}
+
+// Load audio button
+document.getElementById('btn-load')?.addEventListener('click', async () => {
+    const pathInput = document.getElementById('audio-path');
+    const path = pathInput?.value.trim();
+    if (!path) return;
+    try {
+        const res = await fetch('/api/audio/load', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            body: JSON.stringify({ path }),
+        });
+        if (!res.ok) {
+            const err = await res.json();
+            console.error('[audio] load failed:', err.detail);
+            return;
+        }
+        // Wait briefly for mpv to initialize, then fetch waveform
+        setTimeout(loadWaveform, 500);
+    } catch (err) {
+        console.error('[audio] load error:', err);
+    }
+});
+
+// Click on waveform to seek
+document.querySelector('.waveform-container')?.addEventListener('click', (e) => {
+    if (audioDuration <= 0) return;
+    const rect = e.currentTarget.getBoundingClientRect();
+    const pct = (e.clientX - rect.left) / rect.width;
+    const pos = pct * audioDuration;
+    client.send({ type: 'seek', position: pos });
+});
+
+// Resize waveform on window resize
+window.addEventListener('resize', () => {
+    if (waveformPeaks.length > 0) {
+        const canvas = document.getElementById('waveform-canvas');
+        if (canvas) drawWaveform(canvas, waveformPeaks);
+    }
+});
+
 document.getElementById("add-device-form").addEventListener("submit", async (e) => {
     e.preventDefault();
     const form = e.target;

lightsync/frontend/index.html

@@ -48,13 +48,33 @@
         </aside>
 
         <main class="main-area">
-            <div class="panel" id="timeline-panel" style="flex: 1; display: flex; align-items: center; justify-content: center;">
-                <span>[ TIMELINE &#8212; PHASE 2+ ]</span>
+            <div class="panel" id="timeline-panel" style="flex: 1; display: flex; flex-direction: column;">
+                <div class="panel-header">TIMELINE</div>
+                <div class="waveform-container" style="flex: 1; position: relative; min-height: 100px;">
+                    <canvas id="waveform-canvas" style="width: 100%; height: 100%;"></canvas>
+                    <div id="playback-cursor" class="playback-cursor"></div>
+                </div>
             </div>
         </main>
 
         <footer class="transport-bar" id="transport-panel">
-            <span>[ TRANSPORT &#8212; PHASE 2+ ]</span>
+            <div class="transport-controls">
+                <button id="btn-play" class="transport-btn" title="Play">&#9654;</button>
+                <button id="btn-pause" class="transport-btn" title="Pause" style="display:none">&#9646;&#9646;</button>
+                <button id="btn-stop" class="transport-btn" title="Stop">&#9632;</button>
+            </div>
+            <div class="transport-time">
+                <span id="time-current">0:00.0</span>
+                <span class="text-dim">/</span>
+                <span id="time-duration">0:00.0</span>
+            </div>
+            <div class="transport-seek">
+                <input type="range" id="seek-bar" min="0" max="100" value="0" step="0.1" class="seek-slider">
+            </div>
+            <div class="transport-file">
+                <input type="text" id="audio-path" placeholder="Audio file path on server..." class="audio-path-input">
+                <button id="btn-load" class="transport-btn" title="Load audio">LOAD</button>
+            </div>
         </footer>
 
     </div>

lightsync/frontend/style.css

@@ -247,3 +247,93 @@ button:hover {
     color: #ff3333;
     border: none;
 }
+
+/* Transport controls */
+.transport-controls {
+    display: flex;
+    gap: 4px;
+    align-items: center;
+}
+
+.transport-btn {
+    background: var(--bg-panel);
+    color: var(--accent);
+    border: 1px solid var(--border-dim);
+    padding: 4px 10px;
+    font-family: var(--font-mono);
+    font-size: 0.85rem;
+    cursor: pointer;
+    min-width: 36px;
+    text-align: center;
+}
+
+.transport-btn:hover {
+    background: var(--accent);
+    color: var(--bg-primary);
+    border-color: var(--accent);
+}
+
+.transport-time {
+    font-family: var(--font-mono);
+    font-size: 0.85rem;
+    white-space: nowrap;
+    min-width: 120px;
+}
+
+.transport-seek {
+    flex: 1;
+    display: flex;
+    align-items: center;
+}
+
+.seek-slider {
+    width: 100%;
+    accent-color: var(--accent);
+    cursor: pointer;
+}
+
+.transport-file {
+    display: flex;
+    gap: 4px;
+    align-items: center;
+}
+
+.audio-path-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: 0.8rem;
+    width: 200px;
+}
+
+.audio-path-input:focus {
+    border-color: var(--accent);
+    outline: none;
+}
+
+/* Transport bar layout override */
+.transport-bar {
+    gap: 12px;
+    padding: 6px 12px;
+}
+
+/* Waveform */
+.waveform-container {
+    background: var(--bg-primary);
+    border: none;
+    overflow: hidden;
+}
+
+.playback-cursor {
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 2px;
+    height: 100%;
+    background: var(--accent);
+    pointer-events: none;
+    transition: left 0.1s linear;
+    z-index: 10;
+}