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/src/lib.rs

@@ -3,7 +3,6 @@ use axum::{
     routing::{get, post},
     Router,
 };
-use sqlx::sqlite::SqlitePool;
 use tower_http::cors::{CorsLayer, Any};
 use std::sync::Arc;
 
@@ -23,25 +22,12 @@ pub struct AppState {
 pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
     // Initialize logging
     println!("🚀 Starting Calendar Backend Server");
-
-    // Set up database
-    let database_url = std::env::var("DATABASE_URL")
-        .unwrap_or_else(|_| "sqlite:calendar.db?mode=rwc".to_string());
-    
-    let db_pool = SqlitePool::connect(&database_url).await?;
-    
-    // Run migrations - create database file if it doesn't exist
-    // The migrate!() macro looks for migrations in the current directory
-    // so we don't need to run explicit migrations here since we handle it in init_db()
     
     // Create auth service
     let jwt_secret = std::env::var("JWT_SECRET")
         .unwrap_or_else(|_| "your-super-secret-jwt-key-change-in-production".to_string());
     
-    let auth_service = AuthService::new(db_pool, jwt_secret);
-    
-    // Initialize database schema
-    auth_service.init_db().await?;
+    let auth_service = AuthService::new(jwt_secret);
     
     let app_state = AppState { auth_service };
 
@@ -49,7 +35,6 @@ pub async fn run_server() -> Result<(), Box<dyn std::error::Error>> {
     let app = Router::new()
         .route("/", get(root))
         .route("/api/health", get(health_check))
-        .route("/api/auth/register", post(handlers::register))
         .route("/api/auth/login", post(handlers::login))
         .route("/api/auth/verify", get(handlers::verify_token))
         .route("/api/calendar/events", get(handlers::get_calendar_events))