fix(tags): rename-onto-existing in the image modal now merges, not errors

The image-modal tag kebab's rename dialog still showed a leftover stub
('Merging two tags into one lands in FC-2c') on a name collision, dead-ending
the operator. The merge machinery has existed for a while — the Tags view
already resolves rename collisions this way. Wire TagRenameDialog to it: on the
409 collision hint, show the same merge confirmation FandomSetDialog uses
(target name, image associations moved, alias kept) and POST /api/tags/<id>/merge
into the existing tag.

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

frontend/src/components/modal/TagRenameDialog.vue

@@ -2,25 +2,55 @@
   <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>
+      <template v-if="!collision">
+        <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 }}</v-alert>
+      </template>
+
+      <template v-else>
+        <!-- A tag of this (kind, fandom) already has that name. Renaming onto it
+             is a merge, not a fork — same resolution the Tags view offers. -->
+        <v-alert type="warning" variant="tonal" density="compact" class="mb-3">
+          A {{ tag.kind }} tag named “{{ collision.target.name }}” already exists.
+        </v-alert>
+        <p class="text-body-2">
+          Merge “{{ tag.name }}” 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>
+        <v-alert
+          v-if="errorMsg" type="error" variant="tonal" density="compact"
+          class="mt-3"
+        >{{ errorMsg }}</v-alert>
+      </template>
     </v-card-text>
     <v-card-actions>
       <v-spacer />
-      <v-btn @click="$emit('cancel')">Cancel</v-btn>
-      <v-btn
-        color="primary" rounded="pill"
-        :disabled="!newName.trim() || newName === tag.name"
-        :loading="busy" @click="submit"
-      >Rename</v-btn>
+      <template v-if="!collision">
+        <v-btn :disabled="busy" @click="$emit('cancel')">Cancel</v-btn>
+        <v-btn
+          color="primary" rounded="pill"
+          :disabled="!newName.trim() || newName === tag.name"
+          :loading="busy" @click="submit"
+        >Rename</v-btn>
+      </template>
+      <template v-else>
+        <v-btn :disabled="busy" @click="collision = null; errorMsg = null">
+          Back
+        </v-btn>
+        <v-btn
+          color="warning" variant="flat" rounded="pill"
+          :loading="busy" @click="onMerge"
+        >Merge</v-btn>
+      </template>
     </v-card-actions>
   </v-card>
 </template>
@@ -35,22 +65,41 @@ const emit = defineEmits(['renamed', 'cancel'])
 const api = useApi()
 const newName = ref(props.tag.name)
 const errorMsg = ref(null)
-const isCollision = ref(false)
+const collision = ref(null)
 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/${props.tag.id}`, {
       body: { name: newName.value.trim() }
     })
     emit('renamed', updated)
+  } catch (e) {
+    // 409 → the new name collides with an existing tag of the same
+    // (kind, fandom). Offer to merge into it rather than dead-ending.
+    if (e.status === 409 && e.body && e.body.target) {
+      collision.value = e.body
+    } else {
+      errorMsg.value = e.message
+    }
+  } finally {
+    busy.value = false
+  }
+}
+
+async function onMerge() {
+  busy.value = true
+  errorMsg.value = null
+  try {
+    await api.post(`/api/tags/${props.tag.id}/merge`, {
+      body: { target_id: collision.value.target.id }
+    })
+    emit('renamed', { id: collision.value.target.id, name: collision.value.target.name })
   } catch (e) {
     errorMsg.value = e.message
-    isCollision.value = e.status === 409
   } finally {
     busy.value = false
   }