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/hooks/usePitchDetector.ts

@@ -9,7 +9,7 @@ import {
     NOTE_DISPLAY_STABLE_FRAMES,
 } from '../AppConfig';
 
-interface PitchData {
+export interface PitchData {
     frequency: number;
     midi: number;
     note: string;