Clean up obsolete CalDAV environment variables

## Removed Obsolete Environment Variables:
- `CALDAV_SERVER_URL` - provided by user login
- `CALDAV_USERNAME` - provided by user login
- `CALDAV_PASSWORD` - provided by user login
- `CALDAV_TASKS_PATH` - not used in any features

## Kept with Intelligent Discovery:
- `CALDAV_CALENDAR_PATH` - optional override, defaults to smart discovery

## Changes:
### Backend
- Remove `CalDAVConfig::from_env()` method (not used in main app)
- Add `CalDAVConfig::new()` constructor with credentials
- Remove `tasks_path` field from CalDAVConfig
- Update auth service to use new constructor
- Update tests to use hardcoded test values instead of env vars
- Update debug tools to use test credentials

### Frontend
- Remove unused `config.rs` file entirely (frontend uses backend API)

## Current Authentication Flow:
1. User provides CalDAV credentials via login API
2. Backend creates CalDAVConfig dynamically from login request
3. Backend tests authentication via calendar discovery
4. Optional `CALDAV_CALENDAR_PATH` env var can override discovery
5. No environment variables required for normal operation

This simplifies deployment - users only need to provide CalDAV
credentials through the web interface, no server-side configuration required.

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

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

backend/src/auth.rs

@@ -31,13 +31,11 @@ impl AuthService {
         println!("✅ Input validation passed");
 
         // Create CalDAV config with provided credentials
-        let caldav_config = CalDAVConfig {
-            server_url: request.server_url.clone(),
-            username: request.username.clone(),
-            password: request.password.clone(),
-            calendar_path: None,
-            tasks_path: None,
-        };
+        let caldav_config = CalDAVConfig::new(
+            request.server_url.clone(),
+            request.username.clone(), 
+            request.password.clone()
+        );
         println!("📝 Created CalDAV config");
 
         // Test authentication against CalDAV server
@@ -74,13 +72,11 @@ impl AuthService {
     pub fn caldav_config_from_token(&self, token: &str, password: &str) -> Result<CalDAVConfig, ApiError> {
         let claims = self.verify_token(token)?;
         
-        Ok(CalDAVConfig {
-            server_url: claims.server_url,
-            username: claims.username,
-            password: password.to_string(),
-            calendar_path: None,
-            tasks_path: None,
-        })
+        Ok(CalDAVConfig::new(
+            claims.server_url,
+            claims.username, 
+            password.to_string()
+        ))
     }
 
     fn validate_login(&self, request: &CalDAVLoginRequest) -> Result<(), ApiError> {