feat(fc2a): add tag store, TagAutocomplete, and FandomPicker

TagAutocomplete: kind selector + debounced (200ms) search + keyboard nav
(up/down/enter/esc). When the operator types a character that doesn't
exist, the create flow first opens FandomPicker so the new character tag
lands with a valid fandom_id (otherwise the API rejects it).

FandomPicker lists existing fandoms (cached after first load) with an
inline create-new affordance, so the operator never has to leave the modal
to manage fandom hierarchies.

Tag store also exposes a kind→icon→color mapping that the TagPanel reuses
in Task 22 — keeping chip colors visually distinct per kind without per-app
accent collisions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/modal/FandomPicker.vue

@@ -0,0 +1,48 @@
+<template>
+  <v-card>
+    <v-card-title>Pick a fandom</v-card-title>
+    <v-card-text>
+      <v-autocomplete
+        v-model="selectedId"
+        :items="store.fandomCache"
+        :item-title="(f) => f.name"
+        :item-value="(f) => f.id"
+        label="Fandom" clearable density="compact"
+      />
+      <v-divider class="my-3" />
+      <p class="text-caption mb-2">Or create a new fandom:</p>
+      <div class="d-flex" style="gap: 8px;">
+        <v-text-field v-model="newName" placeholder="New fandom name" density="compact" hide-details />
+        <v-btn :disabled="!newName.trim()" rounded="pill" @click="onCreate">Create</v-btn>
+      </div>
+    </v-card-text>
+    <v-card-actions>
+      <v-spacer />
+      <v-btn @click="$emit('cancel')">Cancel</v-btn>
+      <v-btn :disabled="!selectedId" color="primary" rounded="pill" @click="onConfirm">Use this fandom</v-btn>
+    </v-card-actions>
+  </v-card>
+</template>
+
+<script setup>
+import { onMounted, ref } from 'vue'
+import { useTagStore } from '../../stores/tags.js'
+
+const emit = defineEmits(['confirm', 'cancel'])
+const store = useTagStore()
+
+const selectedId = ref(null)
+const newName = ref('')
+
+onMounted(() => { if (store.fandomCache.length === 0) store.loadFandoms() })
+
+async function onCreate() {
+  const f = await store.createFandom(newName.value.trim())
+  selectedId.value = f.id
+  newName.value = ''
+}
+function onConfirm() {
+  const f = store.fandomCache.find(x => x.id === selectedId.value)
+  if (f) emit('confirm', f)
+}
+</script>