[#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>

__test__/components/EventModifications.test.tsx

@@ -302,6 +302,7 @@ describe("CalendarApp integration", () => {
                 end: new Date("2025-11-14T11:31:00.000Z").toISOString(),
                 class: "PUBLIC",
                 partstat: "ACCEPTED",
+                sequence: 2,
                 organizer: {
                   cn: "Alice",
                   cal_address: "alice@example.com",
@@ -380,6 +381,9 @@ describe("CalendarApp integration", () => {
       expect(normalAttendee).toBeTruthy();
       expect(normalAttendee?.partstat).toBe("ACCEPTED");
       expect(normalAttendee?.role).toBe("REQ-PARTICIPANT");
+
+      // Verify SEQUENCE is incremented
+      expect(updatedEvent?.sequence).toBe(3); // 2 + 1
     });
 
     it("changes normal attendee to need action on time update and no organizer changes", async () => {
@@ -432,6 +436,9 @@ describe("CalendarApp integration", () => {
       expect(normalAttendee).toBeTruthy();
       expect(normalAttendee?.partstat).toBe("NEEDS-ACTION");
       expect(normalAttendee?.role).toBe("REQ-PARTICIPANT");
+
+      // Verify SEQUENCE is incremented
+      expect(updatedEvent?.sequence).toBe(3); // 2 + 1
     });
     it("update event attendees on drag", async () => {
       const mockDispatch = jest.fn();
@@ -490,6 +497,9 @@ describe("CalendarApp integration", () => {
         (a: any) => a.cal_address === "bob@example.com"
       );
       expect(normal?.partstat).toBe("NEEDS-ACTION");
+
+      // Verify SEQUENCE is incremented
+      expect(updatedEvent?.sequence).toBe(3); // 2 + 1
     });
 
     it("update event attendees on resize", async () => {
@@ -549,6 +559,9 @@ describe("CalendarApp integration", () => {
         (a: any) => a.cal_address === "bob@example.com"
       );
       expect(normal?.partstat).toBe("NEEDS-ACTION");
+
+      // Verify SEQUENCE is incremented
+      expect(updatedEvent?.sequence).toBe(3); // 2 + 1
     });
   });
 });

__test__/features/Events/eventUtils.test.ts

@@ -837,6 +837,99 @@ describe("calendarEventToJCal", () => {
       ])
     );
   });
+
+  it("includes SEQUENCE field when present in event", () => {
+    const mockEvent = {
+      uid: "event-with-sequence",
+      title: "Event with Sequence",
+      start: "2025-07-23T08:00:00.000Z",
+      end: "2025-07-23T09:00:00.000Z",
+      timezone: "Europe/Paris",
+      allday: false,
+      sequence: 3,
+      attendee: [],
+    };
+
+    const result = calendarEventToJCal(mockEvent as unknown as CalendarEvent);
+
+    const [vevent] = result[2];
+    const props = vevent[1];
+
+    const sequenceProp = props.find((p: any[]) => p[0] === "sequence");
+    expect(sequenceProp).toBeDefined();
+    expect(sequenceProp).toEqual(["sequence", {}, "integer", 3]);
+  });
+
+  it("defaults SEQUENCE to 1 when not present in event", () => {
+    const mockEvent = {
+      uid: "event-without-sequence",
+      title: "Event without Sequence",
+      start: "2025-07-23T08:00:00.000Z",
+      end: "2025-07-23T09:00:00.000Z",
+      timezone: "Europe/Paris",
+      allday: false,
+      attendee: [],
+    };
+
+    const result = calendarEventToJCal(mockEvent as unknown as CalendarEvent);
+
+    const [vevent] = result[2];
+    const props = vevent[1];
+
+    const sequenceProp = props.find((p: any[]) => p[0] === "sequence");
+    expect(sequenceProp).toBeDefined();
+    expect(sequenceProp).toEqual(["sequence", {}, "integer", 1]);
+  });
+
+  it("includes SEQUENCE in recurring event", () => {
+    const mockEvent = {
+      uid: "recurring-event-sequence",
+      title: "Recurring Event with Sequence",
+      start: "2025-07-23T08:00:00.000Z",
+      end: "2025-07-23T09:00:00.000Z",
+      timezone: "Europe/Paris",
+      allday: false,
+      sequence: 2,
+      repetition: { freq: "DAILY", interval: 1 },
+      attendee: [],
+    };
+
+    const result = calendarEventToJCal(mockEvent as unknown as CalendarEvent);
+
+    const [vevent] = result[2];
+    const props = vevent[1];
+
+    const sequenceProp = props.find((p: any[]) => p[0] === "sequence");
+    expect(sequenceProp).toBeDefined();
+    expect(sequenceProp).toEqual(["sequence", {}, "integer", 2]);
+  });
+
+  it("includes SEQUENCE in event exception (with recurrence-id)", () => {
+    const mockEvent = {
+      uid: "event-exception-sequence",
+      title: "Event Exception with Sequence",
+      start: "2025-07-23T08:00:00.000Z",
+      end: "2025-07-23T09:00:00.000Z",
+      timezone: "Europe/Paris",
+      allday: false,
+      sequence: 5,
+      recurrenceId: "2025-07-23T08:00:00Z",
+      attendee: [],
+    };
+
+    const result = calendarEventToJCal(mockEvent as unknown as CalendarEvent);
+
+    const [vevent] = result[2];
+    const props = vevent[1];
+
+    const sequenceProp = props.find((p: any[]) => p[0] === "sequence");
+    expect(sequenceProp).toBeDefined();
+    expect(sequenceProp).toEqual(["sequence", {}, "integer", 5]);
+
+    // Also verify recurrence-id is present
+    const recurrenceIdProp = props.find((p: any[]) => p[0] === "recurrence-id");
+    expect(recurrenceIdProp).toBeDefined();
+  });
 });
 
 describe("combineMasterDateWithFormTime", () => {