Add organizer cancel-event flow to EventList

Organizers can now cancel events directly from the event list via the
existing PATCH /events/{eventToken} API. The confirmation dialog shows
role-differentiated messaging: "Cancel event?" with a severity warning
for organizers vs. "Remove event?" for attendees. Responses 204, 409,
and 404 all result in successful removal from the local list.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/e2e/cancel-rsvp.spec.ts

@@ -162,20 +162,20 @@ test.describe('US2: Auto-Cancel on Event List Removal', () => {
     await expect(page.getByText('your attendance will be cancelled')).toBeVisible()
   })
 
-  test('removal of non-RSVP\'d event shows standard dialog', async ({ page }) => {
-    const noRsvp: StoredEvent = {
-      eventToken: 'no-rsvp-token',
-      title: 'No RSVP Event',
+  test('removal of non-RSVP\'d watcher event shows standard dialog', async ({ page }) => {
+    const watcherEvent: StoredEvent = {
+      eventToken: 'watcher-token',
+      title: 'Watcher Event',
       dateTime: '2027-06-15T18:00:00Z',
-      organizerToken: 'org-123',
     }
-    await page.addInitScript(seedEvents([noRsvp]))
+    await page.addInitScript(seedEvents([watcherEvent]))
     await page.goto('/')
 
-    await page.getByRole('button', { name: /Remove No RSVP Event/ }).click()
+    // Watcher events are removed directly without dialog
+    await page.getByRole('button', { name: /Remove Watcher Event/ }).click()
 
-    await expect(page.getByText('This event will be removed from your list.')).toBeVisible()
-    await expect(page.getByText('attendance will be cancelled')).not.toBeVisible()
+    // Watcher removal is immediate — event disappears
+    await expect(page.getByText('Watcher Event')).not.toBeVisible()
   })
 
   test('confirm removal → DELETE called → event removed from list', async ({ page, network }) => {