Phrase mode: player-style transport, separate display toggles, preview cursor + stepping

- Transport restructured like a music player: Start/Pause, step
  back/forward buttons (while previewing), Preview, Retry, Next.
- Virtual instrument and Hint moved out of the transport row into a
  separate display-toggles row using switch-style toggles (same visual
  language as tuner/metronome switches) — pure show/hide, no run state.
- Preview now tracks a cursor: the current preview event is marked in
  the notation (caret + color via existing currentIdx pipeline).
- Preview stepping: pause-free stepping forward/backward one event at
  a time; each step replays from the cursor position so notes can be
  located on the fretboard. Arrow keys ←/→ work while previewing.
- With Hint active, the fretboard/piano marks the preview cursor note
  (displayIdx follows the preview cursor).
- previewIdx resets on structural changes and when the preview ends.

src/App.tsx

@@ -165,6 +165,12 @@ function App() {
           case 'n':
             phraseRef.current?.next();
             break;
+          case 'arrowleft':
+            phraseRef.current?.previewStep(-1);
+            break;
+          case 'arrowright':
+            phraseRef.current?.previewStep(1);
+            break;
           case 's':
             phraseRef.current?.skip();
             break;
@@ -430,6 +436,7 @@ function App() {
                 <>
                   <div className="help-item"><span>Start / Pause / Resume</span><span className="shortcut-key">Space</span></div>
                   <div className="help-item"><span>Preview phrase (Play / Stop)</span><span className="shortcut-key">P</span></div>
+                  <div className="help-item"><span>While previewing: step note forward / back</span><span className="shortcut-key">→ / ←</span></div>
                   <div className="help-item"><span>Retry phrase</span><span className="shortcut-key">R</span></div>
                   <div className="help-item"><span>New melody / next bars</span><span className="shortcut-key">N</span></div>
                   <div className="help-item"><span>Skip note (at your pace)</span><span className="shortcut-key">S</span></div>