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/app/api/version/route.ts

@@ -0,0 +1,7 @@
+import { NextResponse } from 'next/server'
+
+export const dynamic = 'force-static'
+
+export function GET() {
+  return NextResponse.json({ buildId: process.env.NEXT_PUBLIC_BUILD_ID })
+}