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/debug_caldav.rs

@@ -2,7 +2,12 @@ use crate::calendar::CalDAVClient;
 use crate::config::CalDAVConfig;
 
 pub async fn debug_caldav_fetch() -> Result<(), Box<dyn std::error::Error>> {
-    let config = CalDAVConfig::from_env()?;
+    // Use debug/test configuration
+    let config = CalDAVConfig::new(
+        "https://example.com".to_string(),
+        "debug_user".to_string(),
+        "debug_password".to_string()
+    );
     let client = CalDAVClient::new(config);
     
     println!("=== DEBUG: CalDAV Fetch ===");