Implement complete event creation functionality with CalDAV backend

- Add CalDAV create_event method with proper iCalendar generation
- Add comprehensive backend API for event creation with validation
- Add event creation models and handlers with date/time parsing
- Add frontend service method for creating events via API
- Update frontend to call backend API instead of placeholder
- Fix CalDAV URL construction to avoid duplicate /dav.php paths
- Support all event fields: title, description, dates, location, all-day

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

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

backend/src/models.rs

@@ -70,6 +70,26 @@ pub struct DeleteEventResponse {
     pub message: String,
 }
 
+#[derive(Debug, Deserialize)]
+pub struct CreateEventRequest {
+    pub title: String,
+    pub description: String,
+    pub start_date: String,        // YYYY-MM-DD format
+    pub start_time: String,        // HH:MM format  
+    pub end_date: String,          // YYYY-MM-DD format
+    pub end_time: String,          // HH:MM format
+    pub location: String,
+    pub all_day: bool,
+    pub calendar_path: Option<String>, // Optional - use first calendar if not specified
+}
+
+#[derive(Debug, Serialize)]
+pub struct CreateEventResponse {
+    pub success: bool,
+    pub message: String,
+    pub event_href: Option<String>, // The created event's href/filename
+}
+
 // Error handling
 #[derive(Debug)]
 pub enum ApiError {