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/test/tags.spec.js

@@ -0,0 +1,61 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import { setActivePinia, createPinia } from 'pinia'
+import { useTagStore } from '../src/stores/tags.js'
+
+function stubFetch(handler) {
+  globalThis.fetch = vi.fn(async (url, init) => {
+    const { status, body } = handler(url, init)
+    return {
+      ok: status >= 200 && status < 300,
+      status,
+      statusText: String(status),
+      text: async () => (body == null ? '' : JSON.stringify(body)),
+    }
+  })
+}
+
+describe('tags store: setFandom', () => {
+  beforeEach(() => setActivePinia(createPinia()))
+  afterEach(() => vi.restoreAllMocks())
+
+  it('PATCHes fandom_id, and null to clear', async () => {
+    const s = useTagStore()
+    const calls = []
+    stubFetch((url, init) => {
+      calls.push({ url, init })
+      return { status: 200, body: { id: 5, name: 'Ichigo', kind: 'character', fandom_id: 9 } }
+    })
+    const body = await s.setFandom(5, 9)
+    expect(body.fandom_id).toBe(9)
+    const last = calls.at(-1)
+    expect(last.url).toContain('/api/tags/5')
+    expect(last.init.method).toBe('PATCH')
+    expect(JSON.parse(last.init.body)).toEqual({ fandom_id: 9 })
+
+    await s.setFandom(5, null)
+    expect(JSON.parse(calls.at(-1).init.body)).toEqual({ fandom_id: null })
+  })
+
+  it('sends merge: true when requested', async () => {
+    const s = useTagStore()
+    const calls = []
+    stubFetch((url, init) => {
+      calls.push({ url, init })
+      return { status: 200, body: { id: 2 } }
+    })
+    await s.setFandom(7, 3, { merge: true })
+    expect(JSON.parse(calls.at(-1).init.body)).toEqual({ fandom_id: 3, merge: true })
+  })
+
+  it('throws ApiError carrying the 409 collision body', async () => {
+    const s = useTagStore()
+    stubFetch(() => ({
+      status: 409,
+      body: { error: 'exists', target: { id: 42, name: 'Renji' } },
+    }))
+    await expect(s.setFandom(1, 2)).rejects.toMatchObject({
+      status: 409,
+      body: { target: { id: 42 } },
+    })
+  })
+})