feat: Add persistent session authentication with configurable duration

[michelroegl-brunner]

Summary

This PR implements persistent session authentication with configurable session duration.

Features

• ✅ Persistent session authentication with httpOnly cookies
• ✅ Configurable session duration (1-365 days) in settings
• ✅ Session expiration display in settings modal
• ✅ Logout button next to theme toggle
• ✅ Enhanced token verification with expiration time
• ✅ Retry logic for failed auth checks
• ✅ Comprehensive authentication documentation in help modal
• ✅ Improved session restoration on page load

Changes

• Enhanced AuthProvider with session expiration tracking
• Added session duration configuration in GeneralSettingsModal
• Added logout button in page header
• Updated auth API routes to support session duration
• Added authentication documentation to help modal
• Improved cookie handling and session persistence

Testing

• Sessions persist across page refreshes
• Session duration configuration works correctly
• Logout button appears when authenticated
• Session expiration display updates correctly
• Help documentation is accessible