feat: add system theme mode to follow OS dark/light preference

[eacheat53]

Summary

Add a "System" theme mode that automatically follows the operating system's dark/light preference, alongside the existing Light and Dark options.

Changes

• Theme selector: Replaced the dark theme Toggle with a 3-option <select> dropdown: Light, Dark, and System
• Real-time sync: When set to "System", the extension listens to prefers-color-scheme media query and updates the theme in real-time when the OS switches between light/dark mode
• Default for new installs: Fresh installs now default to "System" mode instead of snapshotting the OS preference once
• Backward compatible: Existing boolean darkTheme values (true/false) continue to work as before
• All 19 locales updated with translated strings for the new options

Files Modified

File	Change
src/types.ts	darkTheme type: boolean → boolean | "system"
src/hooks/useThemeSync.tsx	Added matchMedia listener for system mode
src/options/SectionFlags.tsx	Toggle → select dropdown
src/utils/configUtils.ts	Fresh install defaults to "system"
src/utils/GsmType.ts	Added locale keys
static/locales/*.json	Added translated strings (19 files)