[#318] implement SEQUENCE field for ICS events (#362)

- Add SEQUENCE property to ICS generation (starts at 1 for new events)
- Increment SEQUENCE on event updates (modal, drag & drop, resize)
- Handle SEQUENCE independently for recurring event instances
- Add comprehensive tests for SEQUENCE generation and incrementation
- Ensure proper event ordering for remote calendar participants

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

Co-authored-by: Claude <noreply@anthropic.com>

src/components/Calendar/handlers/eventHandlers.ts

@@ -177,6 +177,7 @@ export const createEventHandlers = (props: EventHandlersProps) => {
         ...event,
         start: computedNewStart.toISOString(),
         end: computedNewEnd.toISOString(),
+        sequence: (event.sequence ?? 1) + 1,
       } as CalendarEvent,
       true
     );
@@ -202,6 +203,7 @@ export const createEventHandlers = (props: EventHandlersProps) => {
                   ...master,
                   start: computedNewStart.toISOString(),
                   end: computedNewEnd.toISOString(),
+                  sequence: (master.sequence ?? 1) + 1,
                 },
               })
             );
@@ -254,6 +256,7 @@ export const createEventHandlers = (props: EventHandlersProps) => {
         ...event,
         start: computedNewStart.toISOString(),
         end: computedNewEnd.toISOString(),
+        sequence: (event.sequence ?? 1) + 1,
       } as CalendarEvent,
       true
     );
@@ -280,6 +283,7 @@ export const createEventHandlers = (props: EventHandlersProps) => {
                   ...master,
                   start: computedNewStart.toISOString(),
                   end: computedNewEnd.toISOString(),
+                  sequence: (master.sequence ?? 1) + 1,
                 },
               })
             );