UI consolidation round: pace checkbox, unified Limit notes section, fixes

- Pace dropdown REMOVED from Phrase setup — replaced by a 'Sync to
  exercise' checkbox in the Metronome widget (analog to 'Auto' in
  single-note modes). pace is derived from the checkbox (checked = tempo,
  unchecked = step).
- Preview clicks now registered in PREVIEW_GROUP: stopping/abandoning a
  preview cancels its scheduled clicks — no more parallel ghost
  metronomes after preview->stop->next exercise.
- Chord pool replaces the chord dropdown entirely (single chord AND
  sequences): togglable degree list, empty = all 7.
- Giuliani figures corrected to NOTE sequences (p always takes the bass/
  root): p-i-m, p-i-m-a, p-a-m-i, a-i-m, p-i-m-a-m-i, p-m-a-m-i-m,
  p-e-a-m-a-m (replaced the fingering-driven patterns that made no
  musical sense).
- 'Limit notes' is now one bordered section like Tuner/Metronome, with a
  header, Clear-all link, and Strings behind a toggle (row hidden until
  restricted).

src/hooks/usePhraseTrainer.ts

@@ -507,11 +507,13 @@ export function usePhraseTrainer(
             }
             // Tempo pace: click along with the preview so the rhythm is
             // audible while listening (quarter beats over the full span).
+            // Registered in PREVIEW_GROUP so stopping the preview also stops
+            // its clicks (no ghost metronomes after an aborted preview).
             if (paceRef.current === 'tempo' && configRef.current.clickSound) {
                 const totalTicks = events.reduce((a, e) => Math.max(a, e.startTick + e.durationTicks), 0);
                 const beats = Math.floor(totalTicks / PPQ);
                 for (let i = 0; i < beats; i++) {
-                    audioEngine.playClickAt(t0 + i * spb, CLICK_VOLUME, i % 4 === 0);
+                    audioEngine.playClickAt(t0 + i * spb, CLICK_VOLUME, i % 4 === 0, PREVIEW_GROUP);
                 }
             }
             setPhaseBoth('preview');