Implement full-screen monthly calendar UI

Added a comprehensive monthly calendar component with modern styling:
- Full monthly view with proper date calculations
- Current day highlighting and navigation
- Responsive design for all screen sizes
- Event indicator support for future integration
- Takes up most of screen space as requested

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/components/mod.rs

@@ -1,5 +1,7 @@
 pub mod login;
 pub mod register;
+pub mod calendar;
 
 pub use login::Login;
-pub use register::Register;
+pub use register::Register;
+pub use calendar::Calendar;