[Feature] Allow users to choose custom alarm sound/music

[mattekudacy]

Description

Allow users to select their preferred alarm sound or music that plays when approaching their destination.

User Story

As a commuter, I want to choose my own alarm sound so I can pick something that will actually wake me up or something less jarring than the default.

Problem

• Default notification sounds may not be loud enough to wake heavy sleepers
• Some users prefer gentler wake-up sounds
• Personalization improves user experience

Proposed Solution

Add sound selection in settings with options:

Features

1. Preset Sounds - Bundle 4-5 built-in alarm sounds
2. Custom Sound - Let users pick audio from their device
3. Preview - Test the sound before saving
4. Volume Control - Separate alarm volume setting

Preset Sound Ideas

Sound	Use Case
Gentle Chime	Light sleepers, quiet environments
Classic Alarm	Standard wake-up
Escalating Beep	Gradually gets louder
Train Announcement	"Next station..." vibe 🚃
Air Horn	Heavy sleepers, guaranteed wake-up

Technical Considerations

• Store bundled sounds in assets/sounds/
• Use audioplayers or just_audio package for playback
• Save preference to SharedPreferences
• For custom sounds: use file_picker to select audio files
• Consider file size limits for custom sounds (e.g., max 1MB)

Acceptance Criteria

• Settings screen with sound selection
• 4-5 bundled preset alarm sounds
• Preview button to test selected sound
• Volume slider for alarm
• Option to select custom audio from device storage
• Persist selection across app restarts
• Selected sound plays on destination approach notification