feat(fc2b): AliasPickerDialog + TagRenameDialog

AliasPicker: debounced autocomplete scoped to the prediction's category
(artist/character map to tag kinds; copyright searches unscoped),
returns the chosen canonical tag id. RenameDialog: PATCH /api/tags/<id>,
surfaces 409 collisions with the FC-2c merge hint.

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

frontend/src/components/modal/AliasPickerDialog.vue

@@ -0,0 +1,63 @@
+<template>
+  <v-card>
+    <v-card-title>Treat as alias for…</v-card-title>
+    <v-card-text>
+      <p class="text-caption mb-2">
+        Pick the existing tag the model's prediction should map to. All
+        future predictions of this name will resolve to your tag.
+      </p>
+      <v-autocomplete
+        v-model="selectedId"
+        :items="results"
+        :item-title="(t) => t.fandom_name ? `${t.name} — ${t.fandom_name}` : t.name"
+        :item-value="(t) => t.id"
+        :loading="loading"
+        label="Canonical tag"
+        no-filter clearable density="compact"
+        @update:search="onSearch"
+      />
+    </v-card-text>
+    <v-card-actions>
+      <v-spacer />
+      <v-btn @click="$emit('cancel')">Cancel</v-btn>
+      <v-btn
+        color="primary" rounded="pill" :disabled="!selectedId"
+        @click="$emit('confirm', selectedId)"
+      >Use this tag</v-btn>
+    </v-card-actions>
+  </v-card>
+</template>
+
+<script setup>
+import { ref } from 'vue'
+import { useApi } from '../../composables/useApi.js'
+
+const props = defineProps({ category: { type: String, required: true } })
+defineEmits(['confirm', 'cancel'])
+
+const api = useApi()
+const results = ref([])
+const loading = ref(false)
+const selectedId = ref(null)
+let debounce = null
+
+function onSearch(q) {
+  if (debounce) clearTimeout(debounce)
+  debounce = setTimeout(async () => {
+    const query = (q || '').trim()
+    if (!query) { results.value = []; return }
+    loading.value = true
+    try {
+      // Scope the autocomplete to the prediction's category where it maps
+      // to a tag kind. 'copyright' has no tag kind; search unscoped there.
+      const kind = ['artist', 'character'].includes(props.category)
+        ? props.category : null
+      const params = { q: query, limit: 20 }
+      if (kind) params.kind = kind
+      results.value = await api.get('/api/tags/autocomplete', { params })
+    } finally {
+      loading.value = false
+    }
+  }, 200)
+}
+</script>

frontend/src/components/modal/TagRenameDialog.vue

@@ -0,0 +1,58 @@
+<template>
+  <v-card>
+    <v-card-title>Rename tag</v-card-title>
+    <v-card-text>
+      <v-text-field
+        v-model="newName" label="New name" density="compact"
+        autofocus @keydown.enter="submit"
+      />
+      <v-alert v-if="errorMsg" type="error" variant="tonal" density="compact" class="mt-2">
+        {{ errorMsg }}
+        <div v-if="isCollision" class="text-caption mt-1">
+          Merging two tags into one lands in FC-2c.
+        </div>
+      </v-alert>
+    </v-card-text>
+    <v-card-actions>
+      <v-spacer />
+      <v-btn @click="$emit('cancel')">Cancel</v-btn>
+      <v-btn
+        color="primary" rounded="pill"
+        :disabled="!newName.trim() || newName === tag.name"
+        :loading="busy" @click="submit"
+      >Rename</v-btn>
+    </v-card-actions>
+  </v-card>
+</template>
+
+<script setup>
+import { ref } from 'vue'
+import { useApi } from '../../composables/useApi.js'
+
+const props = defineProps({ tag: { type: Object, required: true } })
+const emit = defineEmits(['renamed', 'cancel'])
+
+const api = useApi()
+const newName = ref(props.tag.name)
+const errorMsg = ref(null)
+const isCollision = ref(false)
+const busy = ref(false)
+
+async function submit() {
+  if (!newName.value.trim() || newName.value === props.tag.name) return
+  busy.value = true
+  errorMsg.value = null
+  isCollision.value = false
+  try {
+    const updated = await api.patch(`/api/tags/${props.tag.id}`, {
+      body: { name: newName.value.trim() }
+    })
+    emit('renamed', updated)
+  } catch (e) {
+    errorMsg.value = e.message
+    isCollision.value = e.status === 409
+  } finally {
+    busy.value = false
+  }
+}
+</script>