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/stores/tags.js

@@ -49,8 +49,21 @@ export const useTagStore = defineStore('tags', () => {
     return fandom
   }
 
+  // Set / change / clear a character tag's fandom. fandomId null clears it.
+  // Throws ApiError (status 409, body.target) on a name collision in the
+  // target fandom; pass { merge: true } to resolve it by merging this tag
+  // into the existing character. Returns the updated/surviving tag.
+  async function setFandom(tagId, fandomId, { merge = false } = {}) {
+    const body = { fandom_id: fandomId ?? null }
+    if (merge) body.merge = true
+    return await api.patch(`/api/tags/${tagId}`, { body })
+  }
+
   function kindOptions() { return KIND_OPTIONS }
   function colorFor(kind) { return KIND_COLOR[kind] || 'on-surface' }
 
-  return { fandomCache, autocomplete, loadFandoms, createFandom, kindOptions, colorFor }
+  return {
+    fandomCache, autocomplete, loadFandoms, createFandom, setFandom,
+    kindOptions, colorFor
+  }
 })