Modernize settings page with chip design and toggle switches

Replace traditional dropdowns and checkboxes with modern UI components:

Theme, Interval, and Snooze settings:
- Replace select dropdowns with visual chip-based selection
- Add icons for theme options (system, light, dark)
- Compact, modern layout that's easier to scan

Activity Filters and Notifications:
- Replace checkboxes with iOS-style toggle switches
- Add icons and descriptions for better clarity
- Group notification options with clear visual hierarchy

Auto-save improvements:
- Remove "Save Settings" button for cleaner interface
- Add auto-save listeners for all settings
- Show success toasts when settings are saved
- Validate and auto-save token after input

All settings now save instantly as users change them, providing
immediate feedback and a more intuitive experience.

Author: jonmartin721