Use C4 as root note for chord lesson

[Copilot]

Makes chord lesson consistent with intervals lesson by using C4 (MIDI 60) as the only root note.

Changes

• app/types/chords.ts: generateChordPool() now generates 4 challenges (C4 × 4 chord types) instead of 40 (10 root notes × 4 chord types)
• app/hooks/useChordLesson.ts: Reduced CHALLENGES_PER_LESSON from 5 to 4 to match pool size
• Tests: Updated assertions to expect 4 challenges and C4-only root note

Before/After

// Before: 10 root notes
const rootNotes: MidiNote[] = [48, 50, 52, 53, 55, 57, 59, 60, 62, 64];

// After: Single root note
const rootMidi: MidiNote = 60; // C4

Original prompt

Pull Request: #1

Let's always use c4 as the root note

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