feat: Update event modal UI with header icons and improved actions layout

- Add expand and close icons to ResponsiveDialog header in normal mode
- Change 'Show More' button to 'More options' with Add icon
- Hide Cancel button in normal mode, show only in extended mode
- Add showHeaderActions prop to ResponsiveDialog for UI control
- Preserve EventDisplayPreview modal with old UI (no header icons)
- Update all affected test cases to match new UI behavior

__test__/features/Calendars/CalendarModal.test.tsx

@@ -325,9 +325,12 @@ describe("CalendarPopover - Tabs Scenarios", () => {
     const input = screen.getByLabelText(/CalDAV access/i);
     expect(input).toHaveValue("https://cal.example.org/calendars/user1/cal1");
 
-    // Click copy button
-    const copyButton = screen.getAllByRole("button")[0];
-    fireEvent.click(copyButton);
+    // Click copy button (find button containing ContentCopyIcon)
+    const copyIcon = screen.getByTestId("ContentCopyIcon");
+    const copyButton = copyIcon.closest("button");
+    if (copyButton) {
+      fireEvent.click(copyButton);
+    }
 
     expect(navigator.clipboard.writeText).toHaveBeenCalledWith(
       "https://cal.example.org/calendars/user1/cal1"

__test__/features/Events/EventModal.test.tsx

@@ -124,10 +124,10 @@ describe("EventPopover", () => {
     expect(calendarSelect).toBeInTheDocument();
 
     // Check button
-    expect(screen.getByText("Show More")).toBeInTheDocument();
+    expect(screen.getByText("More options")).toBeInTheDocument();
 
     // Extended mode
-    fireEvent.click(screen.getByText("Show More"));
+    fireEvent.click(screen.getByText("More options"));
 
     // Back button appears
     expect(screen.getByLabelText("show less")).toBeInTheDocument();
@@ -330,6 +330,9 @@ describe("EventPopover", () => {
   it("calls onClose when Cancel clicked", () => {
     renderPopover();
 
+    // Cancel button only appears in expanded mode
+    fireEvent.click(screen.getByText("More options"));
+
     fireEvent.click(screen.getByText("Cancel"));
 
     expect(mockOnClose).toHaveBeenCalledWith({}, "backdropClick");