Implement complete event editing functionality with backend update endpoint

Frontend Changes:
- Add edit context menu option to EventContextMenu with pencil icon
- Enhance CreateEventModal to support both create and edit modes
- Add event data conversion methods for pre-populating edit forms
- Implement conditional submit logic (on_create vs on_update callbacks)
- Add update_event method to CalendarService with POST /calendar/events/update

Backend Changes:
- Add UpdateEventRequest and UpdateEventResponse models
- Implement update_event handler with event search by UID across calendars
- Add POST /api/calendar/events/update route
- Full validation and parsing of all event properties for updates
- Integrate with existing CalDAV client update_event functionality

Users can now right-click events, select "Edit Event", modify properties in the modal, and successfully update existing events instead of creating duplicates.

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

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

backend/src/models.rs

@@ -101,6 +101,35 @@ pub struct CreateEventResponse {
     pub event_href: Option<String>, // The created event's href/filename
 }
 
+#[derive(Debug, Deserialize)]
+pub struct UpdateEventRequest {
+    pub uid: String,               // Event UID to identify which event to update
+    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 status: String,            // confirmed, tentative, cancelled
+    pub class: String,             // public, private, confidential
+    pub priority: Option<u8>,      // 0-9 priority level
+    pub organizer: String,         // organizer email
+    pub attendees: String,         // comma-separated attendee emails
+    pub categories: String,        // comma-separated categories
+    pub reminder: String,          // reminder type
+    pub recurrence: String,        // recurrence type
+    pub recurrence_days: Vec<bool>, // [Sun, Mon, Tue, Wed, Thu, Fri, Sat] for weekly recurrence
+    pub calendar_path: Option<String>, // Optional - search all calendars if not specified
+}
+
+#[derive(Debug, Serialize)]
+pub struct UpdateEventResponse {
+    pub success: bool,
+    pub message: String,
+}
+
 // Error handling
 #[derive(Debug)]
 pub enum ApiError {