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/BottomSheet.vue

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+<template>
+  <Teleport to="body">
+    <Transition name="sheet">
+      <div v-if="open" class="sheet-backdrop" @click.self="$emit('close')" @keydown.escape="$emit('close')">
+        <div class="sheet" role="dialog" aria-modal="true" :aria-label="label" ref="sheetEl" tabindex="-1">
+          <div class="sheet__handle" aria-hidden="true" />
+          <slot />
+        </div>
+      </div>
+    </Transition>
+  </Teleport>
+</template>
+
+<script setup lang="ts">
+import { ref, watch, nextTick } from 'vue'
+
+defineProps<{
+  open: boolean
+  label: string
+}>()
+
+defineEmits<{
+  close: []
+}>()
+
+const sheetEl = ref<HTMLElement | null>(null)
+
+watch(
+  () => sheetEl.value,
+  async (el) => {
+    if (el) {
+      await nextTick()
+      const firstInput = el.querySelector<HTMLElement>('input, textarea, button[type="submit"]')
+      if (firstInput) {
+        firstInput.focus()
+      } else {
+        el.focus()
+      }
+    }
+  },
+)
+</script>
+
+<style scoped>
+.sheet-backdrop {
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.4);
+  display: flex;
+  align-items: flex-end;
+  justify-content: center;
+  z-index: 100;
+}
+
+.sheet {
+  background: var(--color-card);
+  border-radius: 20px 20px 0 0;
+  padding: var(--spacing-lg) var(--spacing-xl) var(--spacing-2xl);
+  width: 100%;
+  max-width: var(--content-max-width);
+  display: flex;
+  flex-direction: column;
+  gap: var(--spacing-lg);
+  outline: none;
+}
+
+.sheet__handle {
+  width: 36px;
+  height: 4px;
+  background: #ccc;
+  border-radius: 2px;
+  align-self: center;
+  flex-shrink: 0;
+}
+
+/* Transition */
+.sheet-enter-active,
+.sheet-leave-active {
+  transition: opacity 0.25s ease;
+}
+
+.sheet-enter-active .sheet,
+.sheet-leave-active .sheet {
+  transition: transform 0.25s ease;
+}
+
+.sheet-enter-from,
+.sheet-leave-to {
+  opacity: 0;
+}
+
+.sheet-enter-from .sheet,
+.sheet-leave-to .sheet {
+  transform: translateY(100%);
+}
+</style>

frontend/src/components/RsvpBar.vue

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+<template>
+  <div class="rsvp-bar">
+    <div class="rsvp-bar__inner">
+      <!-- Status state: already RSVPed -->
+      <div v-if="hasRsvp" class="rsvp-bar__status">
+        <span class="rsvp-bar__check" aria-hidden="true">✓</span>
+        <span class="rsvp-bar__text">You're attending!</span>
+      </div>
+
+      <!-- CTA state: no RSVP yet -->
+      <button v-else class="btn-primary rsvp-bar__cta" type="button" @click="$emit('open')">
+        I'm attending
+      </button>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+defineProps<{
+  hasRsvp?: boolean
+}>()
+
+defineEmits<{
+  open: []
+}>()
+</script>
+
+<style scoped>
+.rsvp-bar {
+  position: fixed;
+  bottom: 0;
+  left: 0;
+  right: 0;
+  display: flex;
+  justify-content: center;
+  z-index: 50;
+  padding: var(--spacing-md) var(--content-padding);
+  padding-bottom: calc(var(--spacing-md) + env(safe-area-inset-bottom, 0px));
+}
+
+.rsvp-bar__inner {
+  width: 100%;
+  max-width: var(--content-max-width);
+}
+
+.rsvp-bar__cta {
+  width: 100%;
+}
+
+.rsvp-bar__status {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: var(--spacing-xs);
+  background: var(--color-card);
+  border-radius: var(--radius-card);
+  padding: var(--spacing-md) var(--spacing-lg);
+  box-shadow: var(--shadow-card);
+  font-weight: 600;
+  font-size: 0.95rem;
+  color: var(--color-text);
+}
+
+.rsvp-bar__check {
+  color: #4caf50;
+  font-size: 1.1rem;
+  font-weight: 700;
+}
+
+.rsvp-bar__text {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+</style>

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

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+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()
+  })
+})

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

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+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)
+  })
+})