feat(settings): tag-eval admin card — trigger + persisted report (survives nav)

Frontend for #1130. A maintenance tile in Settings → Tagging:
- Editable concept list + "Run eval" → POST /api/tag-eval (one running at a time).
- Rehydrates on mount via the persisted run (getRun by latest id) and polls while
  running — so the report SURVIVES navigation (operator-flagged); the task runs
  backend-side regardless and the card reconnects to its row.
- Renders the saved report: per-concept head-vs-centroid metrics table (AP/F1/
  precision/recall) with Δ AP, the learning curve (AP @ N positives), and
  thumbnail galleries (head-would-suggest / head-doubts-positive) for eyeballing.

Backend: _examples now stores thumbnail_urls (not just ids) so the report is a
self-contained artifact that renders without per-id lookups on reload.

No new top-level surface — slots into the existing maintenance area.

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

frontend/src/stores/tagEval.js

@@ -0,0 +1,27 @@
+import { defineStore } from 'pinia'
+
+import { useApi } from '../composables/useApi.js'
+
+// Tag-eval (#1130): trigger + revisit the head-vs-centroid learning-curve eval.
+// The run + full report live server-side (tag_eval_run), so the card rehydrates
+// from getRun() on mount — the report survives navigation.
+export const useTagEvalStore = defineStore('tagEval', () => {
+  const api = useApi()
+
+  async function start(params) {
+    return await api.post('/api/tag-eval', { body: { params } })
+  }
+
+  async function getRun(id) {
+    return await api.get(`/api/tag-eval/${id}`)   // includes the full report
+  }
+
+  // The most recent run (light row, no report) — the card calls getRun() with
+  // its id to pull the persisted report on mount.
+  async function latest() {
+    const body = await api.get('/api/tag-eval', { params: { limit: 1 } })
+    return (body.runs && body.runs[0]) || null
+  }
+
+  return { start, getRun, latest }
+})