feat(heads): nightly auto-retrain + inline Retrain button in Explore

Two cadences for keeping heads in sync with your tagging:
- PASSIVE: a nightly `scheduled_train_heads` beat (skips if a run is already
  in flight; creates+commits the run row before dispatching train_heads so the
  ml worker always finds it). Folds the day's accepts/rejects + newly-eligible
  concepts into the heads without anyone clicking.
- ACTIVE: a "Retrain heads" button in the Explore trail bar — bank the +/-
  feedback you just gave while walking content, without a trip to Settings.

Shared logic in a new useHeadTraining composable (trigger + poll + start/finish
toasts), used by the Explore button; reflects an already-running run (incl. the
nightly one) on mount.

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

frontend/src/composables/useHeadTraining.js

@@ -0,0 +1,67 @@
+import { computed, ref } from 'vue'
+
+import { useHeadsStore } from '../stores/heads.js'
+import { toast } from '../utils/toast.js'
+
+// Shared "(re)train the concept heads" behaviour (#114): trigger + poll status,
+// with toasts on start/finish. Backs both the Settings card and the Explore
+// view's inline button so the active retrain works the same everywhere.
+// Call start() on mount and stop() on unmount to manage the poll timer.
+export function useHeadTraining() {
+  const store = useHeadsStore()
+  const summary = ref(null)
+  const busy = ref(false)        // the trigger POST is in flight
+  let timer = null
+
+  const running = computed(() => summary.value?.running_id != null)
+  const headCount = computed(() => summary.value?.head_count ?? 0)
+
+  async function refresh() {
+    try {
+      summary.value = await store.status()
+    } catch { /* non-fatal — the button still offers a fresh train */ }
+  }
+
+  function startPoll() {
+    stopPoll()
+    timer = setInterval(async () => {
+      const wasRunning = running.value
+      await refresh()
+      // Announce completion once, on the running → done transition.
+      if (wasRunning && !running.value) {
+        toast({
+          text: `Heads retrained — ${headCount.value} concept${headCount.value === 1 ? '' : 's'}`,
+          type: 'success',
+        })
+      }
+      if (!running.value) stopPoll()
+    }, 5000)
+  }
+
+  function stopPoll() {
+    if (timer) { clearInterval(timer); timer = null }
+  }
+
+  // Reflect an already-running run (e.g. the nightly one) on mount.
+  async function start() {
+    await refresh()
+    if (running.value) startPoll()
+  }
+
+  async function train() {
+    busy.value = true
+    try {
+      await store.train()
+      toast({ text: 'Head training started…', type: 'info' })
+      await refresh()
+      startPoll()
+    } catch (e) {
+      const msg = e.body?.running_id ? 'Training is already running.' : e.message
+      toast({ text: `Could not start training: ${msg}`, type: 'error' })
+    } finally {
+      busy.value = false
+    }
+  }
+
+  return { summary, running, busy, headCount, refresh, train, start, stop: stopPoll }
+}