Add external calendars feature: display read-only ICS calendars alongside CalDAV calendars

- Database: Add external_calendars table with user relationships and CRUD operations
- Backend: Implement REST API endpoints for external calendar management and ICS fetching
- Frontend: Add external calendar modal, sidebar section with visibility toggles
- Calendar integration: Merge external events with regular events in unified view
- ICS parsing: Support multiple datetime formats, recurring events, and timezone handling
- Authentication: Integrate with existing JWT token system for user-specific calendars
- UI: Visual distinction with 📅 indicator and separate management section

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

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

backend/src/auth.rs

@@ -112,6 +112,17 @@ impl AuthService {
         self.decode_token(token)
     }
 
+    /// Get user from token
+    pub async fn get_user_from_token(&self, token: &str) -> Result<crate::db::User, ApiError> {
+        let claims = self.verify_token(token)?;
+        
+        let user_repo = UserRepository::new(&self.db);
+        user_repo
+            .find_or_create(&claims.username, &claims.server_url)
+            .await
+            .map_err(|e| ApiError::Database(format!("Failed to get user: {}", e)))
+    }
+
     /// Create CalDAV config from token
     pub fn caldav_config_from_token(
         &self,