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('')