feat: version guard — prompt stale clients to refresh after deploys

- Inject NEXT_PUBLIC_BUILD_ID at build time via next.config.ts
- /api/version static route returns current build ID
- VersionGuard client component checks on tab focus + every 5 min
- Shows persistent toast with Refresh button (no auto-reload)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/shared/version-guard.tsx

@@ -0,0 +1,47 @@
+'use client'
+
+import { useEffect, useRef } from 'react'
+import { toast } from 'sonner'
+
+const CLIENT_BUILD_ID = process.env.NEXT_PUBLIC_BUILD_ID
+
+export function VersionGuard() {
+  const notified = useRef(false)
+
+  useEffect(() => {
+    async function checkVersion() {
+      if (notified.current) return
+      try {
+        const res = await fetch('/api/version', { cache: 'no-store' })
+        if (!res.ok) return
+        const { buildId } = await res.json()
+        if (buildId && CLIENT_BUILD_ID && buildId !== CLIENT_BUILD_ID) {
+          notified.current = true
+          toast('A new version is available', {
+            description: 'Refresh to get the latest updates.',
+            duration: Infinity,
+            action: {
+              label: 'Refresh',
+              onClick: () => window.location.reload(),
+            },
+          })
+        }
+      } catch {
+        // Network error — ignore
+      }
+    }
+
+    // Check on tab focus (covers users returning to stale tabs)
+    window.addEventListener('focus', checkVersion)
+
+    // Also check every 5 minutes for long-lived tabs
+    const interval = setInterval(checkVersion, 5 * 60 * 1000)
+
+    return () => {
+      window.removeEventListener('focus', checkVersion)
+      clearInterval(interval)
+    }
+  }, [])
+
+  return null
+}