feat(ui): page édition d'une carte pré-remplie (P3-5, F-06)

Page /cards/:id/edit : charge la carte via cardsApi, pré-remplit CardForm
en mode edit et enregistre via cardsApi.update. Ajoute un test F-06 au
form (pré-remplissage + update + navigation). Coche P3-5 dans TODO.

src/app/cards/[id]/edit/page.tsx

@@ -0,0 +1,45 @@
+import type { Metadata } from 'next'
+import Link from 'next/link'
+import { notFound } from 'next/navigation'
+import { CardForm } from '@/components/card-form'
+import type { Card, CardInput } from '@/domain/card'
+import { ApiClientError, cardsApi } from '@/lib/api-client'
+
+export const metadata: Metadata = { title: 'Modifier la carte' }
+
+export default async function EditCardPage({
+  params,
+}: {
+  params: Promise<{ id: string }>
+}) {
+  const { id } = await params
+  let card: Card | null = null
+  try {
+    card = await cardsApi.get(id)
+  } catch (e) {
+    if (e instanceof ApiClientError && e.status === 404) notFound()
+    throw e
+  }
+
+  const initial: CardInput = {
+    question: card.question,
+    response: card.response,
+    theme: card.theme,
+    tags: card.tags,
+  }
+
+  return (
+    <main className="mx-auto max-w-3xl px-4 py-8">
+      <div className="mb-6">
+        <Link
+          href={`/cards/${id}`}
+          className="text-sm font-medium text-slate-600 underline-offset-4 hover:underline"
+        >
+          ← Retour au détail
+        </Link>
+        <h1 className="mt-2 text-2xl font-bold text-slate-900">{card.question}</h1>
+      </div>
+      <CardForm mode="edit" cardId={card.id} initial={initial} />
+    </main>
+  )
+}