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/views/TagsView.vue

@@ -28,6 +28,7 @@
         v-for="c in store.cards" :key="c.id" :card="c"
         @open="openTag" @rename="onRename" @manage="onManage" @read="onRead"
         @merge-with="onMergeWith" @delete="onDeleteTag"
+        @set-fandom="onSetFandom"
       />
     </div>
 
@@ -85,6 +86,13 @@
         : ''"
       @confirm="onDeleteTagConfirm"
     />
+
+    <v-dialog v-model="fandomDialogOpen" max-width="460">
+      <FandomSetDialog
+        v-if="fandomTarget" :tag="fandomTarget"
+        @updated="onFandomUpdated" @cancel="fandomDialogOpen = false"
+      />
+    </v-dialog>
   </v-container>
 </template>
 
@@ -98,6 +106,7 @@ import { useInfiniteScroll } from '../composables/useInfiniteScroll.js'
 import TagCard from '../components/discovery/TagCard.vue'
 import MergeConfirmDialog from '../components/discovery/MergeConfirmDialog.vue'
 import DestructiveConfirmModal from '../components/modal/DestructiveConfirmModal.vue'
+import FandomSetDialog from '../components/modal/FandomSetDialog.vue'
 
 // Must stay a subset of the backend TagKind enum (character, fandom,
 // general, series, archive, post). 'fandom' is this model's
@@ -140,6 +149,18 @@ function openTag(tagId) {
   router.push({ name: 'gallery', query: { tag_id: tagId } })
 }
 
+// Character fandom editing (dots-menu → FandomSetDialog).
+const fandomDialogOpen = ref(false)
+const fandomTarget = ref(null)
+function onSetFandom(card) {
+  fandomTarget.value = card
+  fandomDialogOpen.value = true
+}
+function onFandomUpdated() {
+  fandomDialogOpen.value = false
+  store.reset()  // reload so the card reflects the new fandom (or its removal)
+}
+
 function onManage(id) {
   router.push({ name: 'series-manage', params: { tagId: id } })
 }