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__/BottomSheet.spec.ts

@@ -0,0 +1,51 @@
+import { describe, it, expect } from 'vitest'
+import { mount } from '@vue/test-utils'
+import BottomSheet from '../BottomSheet.vue'
+
+function mountSheet(open = true) {
+  return mount(BottomSheet, {
+    props: { open, label: 'Test Sheet' },
+    slots: { default: '<p>Sheet content</p>' },
+    attachTo: document.body,
+  })
+}
+
+describe('BottomSheet', () => {
+  it('renders slot content when open', () => {
+    const wrapper = mountSheet(true)
+    expect(document.body.textContent).toContain('Sheet content')
+    wrapper.unmount()
+  })
+
+  it('does not render content when closed', () => {
+    const wrapper = mountSheet(false)
+    expect(document.body.querySelector('[role="dialog"]')).toBeNull()
+    wrapper.unmount()
+  })
+
+  it('has aria-modal and aria-label on the dialog', () => {
+    const wrapper = mountSheet(true)
+    const dialog = document.body.querySelector('[role="dialog"]')!
+    expect(dialog.getAttribute('aria-modal')).toBe('true')
+    expect(dialog.getAttribute('aria-label')).toBe('Test Sheet')
+    wrapper.unmount()
+  })
+
+  it('emits close when backdrop is clicked', async () => {
+    const wrapper = mountSheet(true)
+    const backdrop = document.body.querySelector('.sheet-backdrop')! as HTMLElement
+    await backdrop.click()
+    // Vue test utils tracks emitted events on the wrapper
+    expect(wrapper.emitted('close')).toBeTruthy()
+    wrapper.unmount()
+  })
+
+  it('emits close on Escape key', async () => {
+    const wrapper = mountSheet(true)
+    const backdrop = document.body.querySelector('.sheet-backdrop')! as HTMLElement
+    backdrop.dispatchEvent(new KeyboardEvent('keydown', { key: 'Escape', bubbles: true }))
+    await wrapper.vm.$nextTick()
+    expect(wrapper.emitted('close')).toBeTruthy()
+    wrapper.unmount()
+  })
+})