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>

backend/Cargo.toml

@@ -9,8 +9,6 @@ path = "src/main.rs"
 
 [dependencies]
 # Backend authentication dependencies
-sqlx = { version = "0.7", features = ["runtime-tokio-rustls", "sqlite", "chrono", "uuid", "migrate"] }
-bcrypt = "0.15"
 jsonwebtoken = "9.0"
 tokio = { version = "1.0", features = ["full"] }
 axum = { version = "0.7", features = ["json"] }