MIND-10: Ambient loops with independent volume mix

## Summary
Add 3 ambient audio loops (rain, forest, waves) with independent volume controls.

## Tasks
- [x] Bundle loopable audio files under `assets/`.
- [ ] Add controls for Rain, Forest, and Waves.
- [ ] Implement independent volume sliders for each loop.
- [ ] Persist user volume preferences in local storage.
- [ ] Ensure fade in/out to prevent clicks when toggling.

## Acceptance Criteria
- [ ] Users can toggle each ambient loop independently.
- [ ] Volume sliders work for each track separately.
- [ ] Volume preferences persist across app restarts.
- [ ] Audio loops smoothly without gaps or clicks.