[#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,

src/features/Events/EventUpdateModal.tsx

@@ -604,6 +604,7 @@ function EventUpdateModal({
       organizer: organizer,
       timezone,
       transp: busy,
+      sequence: (event.sequence ?? 1) + 1,
       color: targetCalendar?.color,
       alarm: { trigger: alarm, action: "EMAIL" },
       x_openpass_videoconference: meetingLink || undefined,
@@ -700,6 +701,7 @@ function EventUpdateModal({
           ...newEvent,
           uid: newEventUID,
           URL: `/calendars/${newCalId || calId}/${newEventUID}.ics`,
+          sequence: 1, // New event with new UID starts at sequence 1
         };
 
         // STEP 3: Persist new event to server

src/features/Events/eventUtils.ts

@@ -297,6 +297,7 @@ export function makeVevent(
         formatDateToICal(new Date(event.start), event.allday ?? false, tzid),
       ],
       ["class", {}, "text", event.class ?? "PUBLIC"],
+      ["sequence", {}, "integer", event.sequence ?? 1],
       [
         "x-openpaas-videoconference",
         {},