feat(tags): fandom-edit UI in tags directory + image modal

Adds the missing UI to change a character tag's fandom, in both places:

- FandomSetDialog (shared): pick an existing fandom, create a new one, or
  clear it; on a name collision in the target fandom it surfaces a merge
  confirmation and resolves via setFandom(merge:true). Reuses the tags
  store's fandom cache.
- TagCard kebab gains "Set fandom…" for character tags (→ TagsView opens
  the dialog, reloads on success).
- TagPanel chip kebab gains "Set fandom…" for character tags (→ reloads the
  modal's tag list on success).
- tags store: setFandom(tagId, fandomId, {merge}) action + test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/discovery/TagCard.vue

@@ -55,6 +55,12 @@
               />
             </template>
             <v-list density="compact">
+              <v-list-item
+                v-if="card.kind === 'character'"
+                title="Set fandom…"
+                prepend-icon="mdi-book-open-page-variant"
+                @click="$emit('set-fandom', card)"
+              />
               <v-list-item
                 title="Merge with…"
                 prepend-icon="mdi-call-merge"
@@ -78,7 +84,9 @@
 import { ref } from 'vue'
 
 const props = defineProps({ card: { type: Object, required: true } })
-const emit = defineEmits(['open', 'rename', 'manage', 'read', 'merge-with', 'delete'])
+const emit = defineEmits([
+  'open', 'rename', 'manage', 'read', 'merge-with', 'delete', 'set-fandom',
+])
 
 const editing = ref(false)
 const draft = ref('')

frontend/src/components/modal/FandomSetDialog.vue

@@ -0,0 +1,128 @@
+<template>
+  <v-card>
+    <v-card-title class="text-body-1">Fandom for “{{ tag.name }}”</v-card-title>
+    <v-card-text>
+      <template v-if="!collision">
+        <v-autocomplete
+          v-model="selectedId"
+          :items="store.fandomCache"
+          :item-title="(f) => f.name" :item-value="(f) => f.id"
+          label="Fandom" clearable density="compact"
+          :hint="selectedId == null
+            ? 'No fandom — the character will be unassigned.' : ''"
+          persistent-hint
+        />
+        <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
+            @keydown.enter.prevent="onCreate"
+          />
+          <v-btn
+            :disabled="!newName.trim() || busy" rounded="pill"
+            @click="onCreate"
+          >Create</v-btn>
+        </div>
+        <v-alert
+          v-if="error" type="error" variant="tonal" density="compact"
+          class="mt-3"
+        >{{ error }}</v-alert>
+      </template>
+
+      <template v-else>
+        <v-alert type="warning" variant="tonal" density="compact" class="mb-3">
+          A character named “{{ tag.name }}” already exists in that fandom.
+        </v-alert>
+        <p class="text-body-2">
+          Merge this tag into “{{ collision.target.name }}”?
+          {{ collision.source_image_count }} image
+          association{{ collision.source_image_count === 1 ? '' : 's' }}
+          will move over and this tag will be deleted{{
+            collision.will_alias ? ' (its name kept as a tagger alias)' : '' }}.
+        </p>
+      </template>
+    </v-card-text>
+    <v-card-actions>
+      <v-spacer />
+      <template v-if="!collision">
+        <v-btn variant="text" :disabled="busy" @click="$emit('cancel')">
+          Cancel
+        </v-btn>
+        <v-btn
+          color="primary" rounded="pill" :loading="busy"
+          :disabled="selectedId === (tag.fandom_id ?? null)"
+          @click="onSave"
+        >Save</v-btn>
+      </template>
+      <template v-else>
+        <v-btn variant="text" :disabled="busy" @click="collision = null">
+          Back
+        </v-btn>
+        <v-btn
+          color="warning" variant="flat" rounded="pill" :loading="busy"
+          @click="onConfirmMerge"
+        >Merge</v-btn>
+      </template>
+    </v-card-actions>
+  </v-card>
+</template>
+
+<script setup>
+import { onMounted, ref } from 'vue'
+import { useTagStore } from '../../stores/tags.js'
+
+const props = defineProps({ tag: { type: Object, required: true } })
+const emit = defineEmits(['updated', 'cancel'])
+
+const store = useTagStore()
+
+const selectedId = ref(props.tag.fandom_id ?? null)
+const newName = ref('')
+const busy = ref(false)
+const error = ref(null)
+const collision = ref(null)
+
+onMounted(() => { if (store.fandomCache.length === 0) store.loadFandoms() })
+
+async function onCreate() {
+  const name = newName.value.trim()
+  if (!name) return
+  busy.value = true
+  error.value = null
+  try {
+    const f = await store.createFandom(name)
+    selectedId.value = f.id
+    newName.value = ''
+  } catch (e) {
+    error.value = e.message || String(e)
+  } finally {
+    busy.value = false
+  }
+}
+
+async function save(merge) {
+  busy.value = true
+  error.value = null
+  try {
+    const body = await store.setFandom(props.tag.id, selectedId.value, { merge })
+    emit('updated', body)
+  } catch (e) {
+    // 409 on first attempt → surface the merge confirmation; cross-fandom
+    // collisions can't go through the regular /merge endpoint, so the
+    // resolution is a second setFandom with merge: true.
+    if (!merge && e.status === 409 && e.body && e.body.target) {
+      collision.value = e.body
+    } else {
+      error.value = e.message || String(e)
+      collision.value = null
+    }
+  } finally {
+    busy.value = false
+  }
+}
+
+function onSave() { save(false) }
+function onConfirmMerge() { save(true) }
+</script>

frontend/src/components/modal/TagPanel.vue

@@ -28,6 +28,11 @@
               <v-list-item @click="openRename(tag)">
                 <v-list-item-title>Rename…</v-list-item-title>
               </v-list-item>
+              <v-list-item
+                v-if="tag.kind === 'character'" @click="openSetFandom(tag)"
+              >
+                <v-list-item-title>Set fandom…</v-list-item-title>
+              </v-list-item>
             </v-list>
           </v-menu>
         </span>
@@ -57,6 +62,13 @@
         @renamed="onRenamed" @cancel="renameDialog = false"
       />
     </v-dialog>
+
+    <v-dialog v-model="fandomDialog" max-width="460">
+      <FandomSetDialog
+        v-if="fandomTarget" :tag="fandomTarget"
+        @updated="onFandomUpdated" @cancel="fandomDialog = false"
+      />
+    </v-dialog>
   </aside>
 </template>
 
@@ -67,6 +79,7 @@ import { useTagStore } from '../../stores/tags.js'
 import TagAutocomplete from './TagAutocomplete.vue'
 import SuggestionsPanel from './SuggestionsPanel.vue'
 import TagRenameDialog from './TagRenameDialog.vue'
+import FandomSetDialog from './FandomSetDialog.vue'
 
 const modal = useModalStore()
 const store = useTagStore()
@@ -107,6 +120,18 @@ async function onRenamed() {
   // Reflect the new name in the modal's current tag list without a full reload.
   await modal.reloadTags()
 }
+
+const fandomDialog = ref(false)
+const fandomTarget = ref(null)
+function openSetFandom(tag) {
+  fandomTarget.value = tag
+  fandomDialog.value = true
+}
+async function onFandomUpdated() {
+  fandomDialog.value = false
+  // A fandom change can merge the tag away; reload to reflect the new state.
+  await modal.reloadTags()
+}
 </script>
 
 <style scoped>