Implement watch-event feature (017) with bookmark in RsvpBar

Add client-side watch/bookmark functionality: users can save events to
localStorage without RSVPing via a bookmark button next to the "I'm attending"
CTA. Watched events appear in the event list with a "Watching" label.
Bookmark is only visible for visitors (not attendees or organizers).

Includes spec, plan, research, tasks, unit tests, and E2E tests.

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

frontend/e2e/home-events.spec.ts

@@ -154,12 +154,15 @@ test.describe('US5: Visual Distinction for Event Roles', () => {
     await expect(badge).toHaveClass(/event-card__badge--attendee/)
   })
 
-  test('shows no badge for events without organizerToken or rsvpToken', async ({ page }) => {
+  test('shows watcher badge for events without organizerToken or rsvpToken', async ({ page }) => {
     await page.addInitScript(seedEvents([pastEvent]))
     await page.goto('/')
 
     const card = page.locator('.event-card').filter({ hasText: 'New Year Party' })
-    await expect(card.locator('.event-card__badge')).toHaveCount(0)
+    const badge = card.locator('.event-card__badge')
+    await expect(badge).toBeVisible()
+    await expect(badge).toHaveText('Watching')
+    await expect(badge).toHaveClass(/event-card__badge--watcher/)
   })
 })