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

@@ -0,0 +1,96 @@
+<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>