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/services/calendar_service.rs

@@ -139,6 +139,7 @@ impl CalendarService {
     pub async fn fetch_events_for_month(
         &self, 
         token: &str, 
+        password: &str,
         year: i32, 
         month: u32
     ) -> Result<Vec<CalendarEvent>, String> {
@@ -154,6 +155,9 @@ impl CalendarService {
 
         request.headers().set("Authorization", &format!("Bearer {}", token))
             .map_err(|e| format!("Authorization header setting failed: {:?}", e))?;
+        
+        request.headers().set("X-CalDAV-Password", password)
+            .map_err(|e| format!("Password header setting failed: {:?}", e))?;
 
         let resp_value = JsFuture::from(window.fetch_with_request(&request))
             .await
@@ -407,7 +411,7 @@ impl CalendarService {
     }
 
     /// Refresh a single event by UID from the CalDAV server
-    pub async fn refresh_event(&self, token: &str, uid: &str) -> Result<Option<CalendarEvent>, String> {
+    pub async fn refresh_event(&self, token: &str, password: &str, uid: &str) -> Result<Option<CalendarEvent>, String> {
         let window = web_sys::window().ok_or("No global window exists")?;
         
         let opts = RequestInit::new();
@@ -420,6 +424,9 @@ impl CalendarService {
 
         request.headers().set("Authorization", &format!("Bearer {}", token))
             .map_err(|e| format!("Authorization header setting failed: {:?}", e))?;
+        
+        request.headers().set("X-CalDAV-Password", password)
+            .map_err(|e| format!("Password header setting failed: {:?}", e))?;
 
         let resp_value = JsFuture::from(window.fetch_with_request(&request))
             .await