Add session time tracking with enable/disable toggle

[ManvithReddyyy]

Summary

This PR introduces a new Session Time Tracking feature that displays how long the user has been using Athas during the current session. The timer appears in the Status Bar and updates every second.

Features Added

• New useSessionTime hook for tracking session duration
• Status Bar item showing formatted session time
• Settings toggle to enable/disable session time tracking
• Instant update when toggling the feature
• Does not modify backend or Tauri code

Why This Is Useful

This adds a lightweight productivity metric without impacting performance and integrates cleanly with the existing settings system.

Files Modified

• src/hooks/useSessionTime.ts
• src/features/layout/components/footer/editor-footer.tsx
• src/features/settings/components/tabs/general-settings.tsx

[SnaetWarre]

This could be an extension.

I don't think this is a place for in the main build...