fix(notifications): use collapsible for advanced sound settings

[tyulyukov]

Summary

• Replaced inline Switch component for advanced sound settings with a collapsible button labeled "Customize per event"
• Advanced per-event sound configuration is now hidden by default in a Collapsible component, reducing visual clutter
• Main "Notification sound" section displays the unified sound picker without overwhelming users who don't need per-event customization
• Added animated chevron icon that rotates based on collapsible open/closed state for clearer affordance
• Improved visual hierarchy by introducing a border separator above the collapsible toggle

Testing

• Open Settings > Notifications > Sound mode
• Verify "Notification sound" section displays with unified sound picker
• Verify "Customize per event" button is visible below the main section
• Click "Customize per event" button and verify the section expands smoothly, showing per-event sound controls
• Click again to collapse and verify chevron rotates
• Verify reset button still works correctly and resets all sound settings
• Test sound preview functionality for both unified and per-event sounds
• Verify collapsible state persists correctly during UI navigation