feat(ST-1134): LandscapeGuard component (T-319)

Toont rotate-overlay in portrait, niets in landscape. Kinderen blijven altijd
in DOM — geen unmount zodat SSE-streams overleven bij rotatie.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

components/mobile/landscape-guard.tsx

@@ -0,0 +1,32 @@
+'use client'
+
+import { useEffect, useState } from 'react'
+import { RotateCw } from 'lucide-react'
+
+export function LandscapeGuard({ children }: { children: React.ReactNode }) {
+  const [isPortrait, setIsPortrait] = useState(false)
+
+  useEffect(() => {
+    const mq = window.matchMedia('(orientation: portrait)')
+    const update = () => setIsPortrait(mq.matches)
+    update()
+    mq.addEventListener('change', update)
+    return () => mq.removeEventListener('change', update)
+  }, [])
+
+  return (
+    <>
+      {children}
+      {isPortrait && (
+        <div
+          role="alert"
+          aria-live="assertive"
+          className="fixed inset-0 z-50 flex flex-col items-center justify-center gap-4 bg-background text-foreground p-6"
+        >
+          <RotateCw className="size-12 text-primary" />
+          <p className="text-base font-medium text-center">Draai je telefoon naar landscape</p>
+        </div>
+      )}
+    </>
+  )
+}