Add update-available banner + versioned /health endpoint

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/UpdateBanner.tsx

@@ -0,0 +1,44 @@
+import { RefreshCw } from 'lucide-react'
+
+export function UpdateBanner({ visible }: { visible: boolean }) {
+  if (!visible) return null
+  return (
+    <div style={{
+      position: 'fixed',
+      bottom: 0,
+      left: 0,
+      right: 0,
+      zIndex: 9999,
+      background: 'var(--sidebar)',
+      color: 'var(--text-light)',
+      display: 'flex',
+      alignItems: 'center',
+      justifyContent: 'center',
+      gap: '12px',
+      padding: '10px 16px',
+      fontSize: '14px',
+      boxShadow: '0 -2px 8px rgba(0,0,0,0.3)',
+    }}>
+      <span>A new version is available.</span>
+      <button
+        onClick={() => window.location.reload()}
+        style={{
+          display: 'flex',
+          alignItems: 'center',
+          gap: '6px',
+          background: 'var(--accent)',
+          color: 'var(--sidebar)',
+          border: 'none',
+          borderRadius: '4px',
+          padding: '6px 14px',
+          fontWeight: 600,
+          cursor: 'pointer',
+          fontSize: '13px',
+        }}
+      >
+        <RefreshCw size={14} strokeWidth={1.75} />
+        Reload
+      </button>
+    </div>
+  )
+}