feat(06-01): add YouTube URL input with terminal progress bar

- Add yt-url input, FETCH button, and yt-progress span to transport bar
- Add fetchWithProgress() SSE ReadableStream reader function
- Add renderProgressBar() terminal-style block fill renderer (DOWNLOADING ████)
- Wire FETCH button click handler with D-02 disable-during-download behavior
- Auto-load audio into player after download (D-01 same pipeline as upload)
- Add .yt-url-input and .yt-progress CSS styles (flat, monospace, no border-radius)
- Enable yt-url input on page load via JS

lightsync/frontend/style.css

@@ -553,3 +553,29 @@ button:hover {
     white-space: nowrap;
     flex-shrink: 0;
 }
+
+/* YouTube URL input (Phase 6) */
+.yt-url-input {
+    background: var(--bg-panel);
+    border: 1px solid var(--border-dim);
+    color: var(--text-primary);
+    font-family: var(--font-mono);
+    font-size: 12px;
+    padding: 2px 6px;
+    width: 180px;
+    outline: none;
+}
+.yt-url-input:focus {
+    border-color: var(--accent);
+}
+.yt-url-input:disabled {
+    opacity: 0.4;
+    cursor: not-allowed;
+}
+.yt-progress {
+    font-family: var(--font-mono);
+    font-size: 11px;
+    color: var(--accent);
+    white-space: nowrap;
+    min-width: 200px;
+}