Add settings and UI enhancements

New Features:
- Add dedicated options page with theme, dimensions, and display settings
- Implement dark/light/auto theme support with system preference detection
- Add customizable popup width (400-1200px) and height (300-800px)
- Add adjustable font size setting (0.7-2.0rem)
- Add close button (×) for each tab with hover/keyboard selection visibility
- Add audio indicator (🔊) for tabs playing sound
- Add favicon display with toggle option (default: on)
- Implement fixed search input with gradient fade effect

Search Improvements:
- Search now matches both tab titles and URLs
- Add special "audio" and "sound" keywords to filter audible tabs

Technical Changes:
- Switch from storage.sync to storage.local API for better compatibility
- Add explicit addon ID to manifest for storage API support
- Convert hardcoded styles to use CSS custom properties and rem units
- Real-time settings updates for the popup

Change the default command shortcut to Ctrl+Space

- Enable Firefox 57 compatibility
- Change the badge color based on the number of open tabs
- Focus on the filter input if anything but a tab link is clicked

Fix the keyboard shortcut so it uses Ctrl rather than Cmd on Macs