feat: File placement card — survey, plan per artist, apply, put back (4246, slice 3c)

The UI half of the reconciler, in Maintenance. Survey shows how many images
sit in the wrong artist's folder and which folders they are in; each artist
gets its own Plan button; each run can be reviewed, applied, and put back.

Deliberately NOT using useMaintenanceTask. That composable stashes a task id
in localStorage so a result survives navigate-away, which is the right answer
when the only record is a Celery result. Here the runs are database rows — so
a reload, another machine, or coming back tomorrow simply shows the same
state, because the state IS the row. The card polls the runs endpoint instead.

Copy avoids the vocabulary this work has been tripping over: "folder", "put
back", "in the wrong folder" rather than artist_id, revert and canonical. The
one thing the operator most needs to know — nothing here changes who an image
belongs to — is what the blurb says first.

Three things I had assumed and checked instead: MaintenanceTile lives in
common/ not settings/; there is no generic ConfirmDialog (BackupCard uses a
purpose-built modal), so this uses a plain v-dialog; and `loadArtistNames`
did not exist — it does now, mapping id to name so a run row reads "Conto"
rather than "#47". The name deliberately is not denormalised into the run: it
belongs to the artist and would go stale on a rename.

Also extracted `stubFetch` to frontend/test/stubFetch.js. The shape ledger
flagged it as a byte-identical duplicate across five specs and this would
have been the sixth; the others keep their copies until each is next touched.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/test/stubFetch.js

@@ -0,0 +1,24 @@
+import { vi } from 'vitest'
+
+// The canonical fetch stub for store specs.
+//
+// `handler(url, init)` returns `{ status, body }`; body is JSON-encoded, and
+// `ok` is derived from the status so a store's error path can be exercised by
+// returning 4xx/5xx. Returns the vi.fn so a caller can assert on calls.
+//
+// Extracted 2026-09-21 from six specs carrying byte-identical copies
+// (adminStore, credentials, dbMaintenance, gallery, suggestions,
+// galleryRelatedStrip). Those still hold their own; migrate each the next
+// time it is touched rather than in one sweep.
+export 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)),
+    }
+  })
+  return globalThis.fetch
+}