feat: recorded affirmations with music backing

[eddmann]

Overview

Allow users to record personal affirmations/positive scripts. The recording gets layered with user-selected background music and plays continuously during breathing sessions, replacing the text affirmations.

User Flow

1. Settings → "Record Affirmations"

• Free-form recording - no time limit, record what feels right
• Preview playback before saving

2. Music Selection

• Pick from existing ambient tracks to layer behind the recording
• Preview the mix before saving

3. During Breathing Session

• Recorded audio + music plays continuously (loops if needed)
• Replaces text affirmations entirely
• Still works alongside chimes and haptics

Technical Notes

• Leverages existing MusicController for background tracks
• Extends experimental voice recording work from conductor/voice-countdown branch
• Audio mixing: voice at foreground level, music ducked underneath