Feedback round 2: strings filter, key sync, fixed clef, unified metronome

- Strings filter: per-string toggle buttons (replaces fret-window-only
  semantics as a third 'limits notes' control); combines with fret window
  and key filter in targetPool (fretted instruments).
- Key sync checkbox: display key signature can follow the target key
  (major/minor mapping), default off — both remain independently settable.
- LiveNoteStaff: optional fixed clef — guitar/bass now keep their usual
  clef instead of flipping at middle C; octave markers handle range.
- Unified metronome widget (MetronomeWidget) with animated linear pendulum
  synced to the audible click; shown in ALL modes next to the tuner.
  Phrase BPM now reads from settings.rhythm.bpm (single source); the
  phrase-local BPM control was removed from the setup panel.
- Tests: 160/160; tsc + eslint clean; build ok.

src/App.tsx

@@ -332,6 +332,7 @@ function App() {
                 midi={displayedPitch ? displayedPitch.midi : null}
                 transpose={resolveClefTranspose(currentInstrumentDef, settings.difficulty).transpose}
                 keySignature={settings.keySignature}
+                clef={currentInstrumentDef.clefMode === 'treble' ? 'treble' : currentInstrumentDef.clefMode === 'bass' ? 'bass' : undefined}
                 theme={settings.theme}
               />
               <div className={`pitch-readout ${displayedPitch ? 'active' : ''}`}>