Add RSVP frontend: bottom sheet form, RsvpBar, and localStorage persistence

Introduces BottomSheet and RsvpBar components, integrates the RSVP
submission flow into EventDetailView, extends useEventStorage with
saveRsvp/getRsvp, and adds unit tests plus an E2E spec for the RSVP
workflow.

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

frontend/src/components/__tests__/RsvpBar.spec.ts

@@ -0,0 +1,30 @@
+import { describe, it, expect } from 'vitest'
+import { mount } from '@vue/test-utils'
+import RsvpBar from '../RsvpBar.vue'
+
+describe('RsvpBar', () => {
+  it('renders CTA button when hasRsvp is false', () => {
+    const wrapper = mount(RsvpBar)
+    expect(wrapper.find('.rsvp-bar__cta').exists()).toBe(true)
+    expect(wrapper.find('.rsvp-bar__cta').text()).toBe("I'm attending")
+    expect(wrapper.find('.rsvp-bar__status').exists()).toBe(false)
+  })
+
+  it('renders status text when hasRsvp is true', () => {
+    const wrapper = mount(RsvpBar, { props: { hasRsvp: true } })
+    expect(wrapper.find('.rsvp-bar__status').exists()).toBe(true)
+    expect(wrapper.find('.rsvp-bar__text').text()).toBe("You're attending!")
+    expect(wrapper.find('.rsvp-bar__cta').exists()).toBe(false)
+  })
+
+  it('emits open when CTA button is clicked', async () => {
+    const wrapper = mount(RsvpBar)
+    await wrapper.find('.rsvp-bar__cta').trigger('click')
+    expect(wrapper.emitted('open')).toHaveLength(1)
+  })
+
+  it('does not render CTA button when hasRsvp is true', () => {
+    const wrapper = mount(RsvpBar, { props: { hasRsvp: true } })
+    expect(wrapper.find('button').exists()).toBe(false)
+  })
+})