Implement calendar deletion with right-click context menu

Added complete calendar deletion functionality including:
- Context menu component with right-click activation on calendar items
- Backend API endpoint for calendar deletion with CalDAV DELETE method
- Frontend integration with calendar list refresh after deletion
- Fixed URL construction to prevent double /dav.php path issue
- Added proper error handling and user feedback

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

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

backend/src/models.rs

@@ -47,6 +47,17 @@ pub struct CreateCalendarResponse {
     pub message: String,
 }
 
+#[derive(Debug, Deserialize)]
+pub struct DeleteCalendarRequest {
+    pub path: String,
+}
+
+#[derive(Debug, Serialize)]
+pub struct DeleteCalendarResponse {
+    pub success: bool,
+    pub message: String,
+}
+
 // Error handling
 #[derive(Debug)]
 pub enum ApiError {