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/tests/integration_tests.rs

@@ -72,9 +72,9 @@ mod test_utils {
         
         pub async fn login(&self) -> String {
             let login_payload = json!({
-                "username": std::env::var("CALDAV_USERNAME").unwrap_or("test".to_string()),
-                "password": std::env::var("CALDAV_PASSWORD").unwrap_or("test".to_string()),
-                "server_url": std::env::var("CALDAV_SERVER_URL").unwrap_or("https://example.com".to_string())
+                "username": "test".to_string(),
+                "password": "test".to_string(),
+                "server_url": "https://example.com".to_string()
             });
             
             let response = self.client
@@ -134,12 +134,10 @@ mod tests {
     async fn test_auth_login() {
         let server = TestServer::start().await;
         
-        // Load credentials from .env 
-        dotenvy::dotenv().ok();
-        let username = std::env::var("CALDAV_USERNAME").unwrap_or("test".to_string());
-        let password = std::env::var("CALDAV_PASSWORD").unwrap_or("test".to_string());
-        
-        let server_url = std::env::var("CALDAV_SERVER_URL").unwrap_or("https://example.com".to_string());
+        // Use test credentials
+        let username = "test".to_string();
+        let password = "test".to_string(); 
+        let server_url = "https://example.com".to_string();
         
         let login_payload = json!({
             "username": username,
@@ -196,7 +194,7 @@ mod tests {
         
         // Load password from env for CalDAV requests
         dotenvy::dotenv().ok();
-        let password = std::env::var("CALDAV_PASSWORD").unwrap_or("test".to_string());
+        let password = "test".to_string();
         
         let response = server.client
             .get(&format!("{}/api/user/info", server.base_url))
@@ -226,7 +224,7 @@ mod tests {
         
         // Load password from env for CalDAV requests  
         dotenvy::dotenv().ok();
-        let password = std::env::var("CALDAV_PASSWORD").unwrap_or("test".to_string());
+        let password = "test".to_string();
         
         let response = server.client
             .get(&format!("{}/api/calendar/events?year=2024&month=12", server.base_url))
@@ -254,7 +252,7 @@ mod tests {
         
         // Load password from env for CalDAV requests
         dotenvy::dotenv().ok(); 
-        let password = std::env::var("CALDAV_PASSWORD").unwrap_or("test".to_string());
+        let password = "test".to_string();
         
         let create_payload = json!({
             "title": "Integration Test Event",
@@ -308,7 +306,7 @@ mod tests {
         
         // Load password from env for CalDAV requests
         dotenvy::dotenv().ok();
-        let password = std::env::var("CALDAV_PASSWORD").unwrap_or("test".to_string());
+        let password = "test".to_string();
         
         // Use a dummy UID for testing - this will likely return 404 but we're testing the endpoint structure
         let test_uid = "test-event-uid";
@@ -373,7 +371,7 @@ mod tests {
         
         // Load password from env for CalDAV requests
         dotenvy::dotenv().ok();
-        let password = std::env::var("CALDAV_PASSWORD").unwrap_or("test".to_string());
+        let password = "test".to_string();
         
         let create_payload = json!({
             "title": "Integration Test Series",
@@ -431,7 +429,7 @@ mod tests {
         
         // Load password from env for CalDAV requests
         dotenvy::dotenv().ok();
-        let password = std::env::var("CALDAV_PASSWORD").unwrap_or("test".to_string());
+        let password = "test".to_string();
         
         let update_payload = json!({
             "series_uid": "test-series-uid",
@@ -493,7 +491,7 @@ mod tests {
         
         // Load password from env for CalDAV requests
         dotenvy::dotenv().ok();
-        let password = std::env::var("CALDAV_PASSWORD").unwrap_or("test".to_string());
+        let password = "test".to_string();
         
         let delete_payload = json!({
             "series_uid": "test-series-to-delete",