Add Assist mode: primitive note display for practicing from paper

New game mode 'assist' — no trainer, no scoring:
- Large display of the currently mic-detected note
- Live notation staff (same pipeline as the pitch monitor)
- Live fretboard showing alternative positions for the played note
  (respects the fret-window practice filter)
- Tuner and Metronome remain the shared footer tools, identical in
  all modes; user can practice own-pace from paper with a free
  metronome

Header mode toggle gains 'Assist'; 'm' shortcut cycles back to sight
reading from assist/phrase. Export PitchData type.

src/types/SettingsTypes.ts

@@ -134,7 +134,7 @@ export interface AppSettings {
     instrument: string;
     rhythm: RhythmSettings;
     zenMode: boolean;
-    gameMode: 'sight_reading' | 'ear_training' | 'phrase';
+    gameMode: 'sight_reading' | 'ear_training' | 'phrase' | 'assist';
     customMinFret?: number;
     customMaxFret?: number;
     autoPlaySightReading?: boolean;