[#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/features/Events/EventModal.tsx

@@ -733,6 +733,7 @@ function EventPopover({
         },
       ],
       transp: busy,
+      sequence: 1,
       color: targetCalendar?.color,
       alarm: { trigger: alarm, action: "EMAIL" },
       x_openpass_videoconference: meetingLink || undefined,