feat(heads): admin card to train + inspect concept heads (#114 B)

The UI for the heads subsystem: Settings → Tagging → "Concept heads". Shows
head count, auto-apply-ready count, and last-trained; a Train/Retrain button
(one run at a time, polls while running, surfaces a failed run's error); an
empty state guiding the operator to tag first; and a per-concept table (name,
category, +tags, AP, P, R, auto-apply ⚡) sorted strongest-first so weak/under-
tagged concepts are obvious. Rehydrates status from GET /api/heads on mount so
it survives navigation. Pulls head_min_positives from ML settings for copy.

Slice C (swap the rail's suggestions to heads, remove Camie + centroid) is next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

frontend/src/stores/heads.js

@@ -0,0 +1,24 @@
+import { defineStore } from 'pinia'
+
+import { useApi } from '../composables/useApi.js'
+
+// Heads (#114): the per-concept classifiers that LEARN from your tags and power
+// suggestions (replacing Camie + centroid). Training runs as a background task;
+// the card rehydrates status from GET /api/heads on mount so it survives
+// navigation (the run lives in head_training_run server-side).
+export const useHeadsStore = defineStore('heads', () => {
+  const api = useApi()
+
+  // Summary: head_count, graduated_count, last_trained_at, running_id, the
+  // per-concept head table, and recent training runs.
+  async function status() {
+    return await api.get('/api/heads')
+  }
+
+  // (Re)train all eligible heads. One run at a time (409 if already running).
+  async function train(params = {}) {
+    return await api.post('/api/heads/train', { body: { params } })
+  }
+
+  return { status, train }
+})