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/RsvpBar.vue

@@ -0,0 +1,75 @@
+<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>