Refactor authentication from database to direct CalDAV authentication

Major architectural change to simplify authentication by authenticating directly against CalDAV servers instead of maintaining a local user database.

Backend changes:
- Remove SQLite database dependencies and user storage
- Refactor AuthService to authenticate directly against CalDAV servers
- Update JWT tokens to store CalDAV server info instead of user IDs
- Implement proper CalDAV calendar discovery with XML parsing
- Fix URL construction for CalDAV REPORT requests
- Add comprehensive debug logging for authentication flow

Frontend changes:
- Add server URL input field to login form
- Remove registration functionality entirely
- Update calendar service to pass CalDAV passwords via headers
- Store CalDAV credentials in localStorage for API calls

Key improvements:
- Simplified architecture eliminates database complexity
- Direct CalDAV authentication ensures credentials always work
- Proper calendar discovery automatically finds user calendars
- Robust error handling and debug logging for troubleshooting

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

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

src/components/mod.rs

@@ -1,9 +1,7 @@
 pub mod login;
-pub mod register;
 pub mod calendar;
 pub mod event_modal;
 
 pub use login::Login;
-pub use register::Register;
 pub use calendar::Calendar;
 pub use event_modal::EventModal;