Metronome rewrite: single sound checkbox, clean click architecture

- MetronomeWidget rebuilt cleanly: ONE 'Click sound' checkbox (the earlier
  restructure left duplicated option rows behind — neither worked). Widget
  body always visible (dimmed when disarmed); toggle arms sound+pendulum.
- Click decision unified: 'sync to metronome' derives phrase clickSound
  from the widget's sound checkbox (no separate phrase click option);
  'at your pace' clicks when the metronome is armed.
- Chord selection: abwählbare Liste (arpeggioChordSelection) restricts
  progression chords; first bar draws a random chord FROM THE SELECTION
  (fixed first chord repeated the exercise identically).
- Scales: rhythm setting (quarters/eighths) shared with arpeggios;
  drill id includes rhythm to invalidate cached scores.
- Beam fix: register Beam via note.setBeam so VexFlow suppresses flags
  (eighths/16ths previously drew BOTH beam and flag).

src/types/SettingsTypes.ts

@@ -53,8 +53,11 @@ export interface PhraseSettings {
     rhythmLevel: 1 | 2 | 3;
     scaleCoverage: 'one-octave' | 'two-octave' | 'position';
     scaleDirection: 'up' | 'down' | 'updown';
+    scaleRhythm: 'quarters' | 'eighths';
     /** Arpeggio settings (material = 'arpeggio'). */
-    arpeggioDegree: string; // ArpeggioDegree
+    arpeggioDegree: string; // ArpeggioDegree (single-chord mode / sequence start)
+    /** Sequence mode: which degrees may appear (empty = all 7). */
+    arpeggioChordSelection: string[];
     arpeggioPattern: 'up' | 'down' | 'updown' | '1235';
     arpeggioCoverage: 'one-octave' | 'two-octave';
     /** Sequence mode: bars per exercise (1 = single chord). */
@@ -90,7 +93,9 @@ export const DEFAULT_PHRASE_SETTINGS: PhraseSettings = {
     rhythmLevel: 1,
     scaleCoverage: 'one-octave',
     scaleDirection: 'updown',
+    scaleRhythm: 'quarters',
     arpeggioDegree: 'I',
+    arpeggioChordSelection: [],
     arpeggioPattern: 'up',
     arpeggioCoverage: 'one-octave',
     arpeggioBars: 1,