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/App.css

@@ -1126,3 +1126,49 @@ button {
     justify-content: safe center;
     justify-content: center;
 }
+
+/* ---- Assist Mode ------------------------------------------------------- */
+/* Primitive practice companion: big detected note + live staff + optional
+   fretboard with alternative positions. No trainer actions. */
+
+.assist-card {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: var(--spacing-md);
+  padding: var(--spacing-lg);
+}
+
+.assist-note-area {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 110px;
+}
+
+.assist-note-name {
+  font-size: 5rem;
+  font-weight: 800;
+  line-height: 1;
+  color: var(--color-text-main);
+  font-variant-emoji: text;
+}
+
+.assist-note-name.placeholder {
+  color: var(--color-text-muted);
+  opacity: 0.5;
+}
+
+.assist-fretboard {
+  width: 100%;
+  display: flex;
+  justify-content: center;
+  overflow-x: auto;
+}
+
+.assist-hint {
+  font-size: 12px;
+  color: var(--color-text-muted);
+  margin: 0;
+  text-align: center;
+}