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>

src/components/event_context_menu.rs

@@ -15,6 +15,7 @@ pub struct EventContextMenuProps {
     pub x: i32,
     pub y: i32,
     pub event: Option<CalendarEvent>,
+    pub on_edit: Callback<()>,
     pub on_delete: Callback<DeleteAction>,
     pub on_close: Callback<()>,
 }
@@ -37,6 +38,15 @@ pub fn event_context_menu(props: &EventContextMenuProps) -> Html {
         .map(|event| event.recurrence_rule.is_some())
         .unwrap_or(false);
 
+    let on_edit_click = {
+        let on_edit = props.on_edit.clone();
+        let on_close = props.on_close.clone();
+        Callback::from(move |_: MouseEvent| {
+            on_edit.emit(());
+            on_close.emit(());
+        })
+    };
+
     let create_delete_callback = |action: DeleteAction| {
         let on_delete = props.on_delete.clone();
         let on_close = props.on_close.clone();
@@ -52,6 +62,10 @@ pub fn event_context_menu(props: &EventContextMenuProps) -> Html {
             class="context-menu" 
             style={style}
         >
+            <div class="context-menu-item" onclick={on_edit_click}>
+                <span class="context-menu-icon">{"✏️"}</span>
+                {"Edit Event"}
+            </div>
             {
                 if is_recurring {
                     html! {