Feature add error page (新機能：エラーページの追加)

src/app/error.module.css

@@ -0,0 +1,81 @@
+.container {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 100vh;
+  padding: 2rem;
+  text-align: center;
+  background: var(--background);
+}
+
+.card {
+  width: 100%;
+  max-width: 460px;
+  padding: 2rem;
+  border-radius: 18px;
+  background: var(--background);
+  border: 1px solid #e5e7eb;
+  box-shadow: 0 20px 45px rgba(15, 23, 42, 0.08);
+}
+
+.title {
+  margin: 0 0 1rem;
+  font-size: 1.75rem;
+  line-height: 1.1;
+  color: var(--foreground);
+}
+
+.description {
+  margin: 0 0 1.75rem;
+  color: #475569;
+  line-height: 1.75;
+}
+
+.button {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 100%;
+  min-height: 44px;
+  padding: 0.9rem 1.25rem;
+  border: none;
+  border-radius: 10px;
+  background-color: #2563eb;
+  color: #ffffff;
+  font-size: 1rem;
+  font-weight: 600;
+  cursor: pointer;
+  transition: background-color 0.2s ease;
+}
+
+.button:hover:not(:disabled) {
+  background-color: #1d4ed8;
+}
+
+.button:focus-visible {
+  outline: 3px solid #2563eb;
+  outline-offset: 2px;
+}
+
+@media (prefers-color-scheme: dark) {
+  .card {
+    border-color: #444;
+    box-shadow: 0 20px 45px rgba(0, 0, 0, 0.5);
+  }
+
+  .description {
+    color: #aaa;
+  }
+
+  .button {
+    background-color: #3b82f6;
+  }
+
+  .button:hover:not(:disabled) {
+    background-color: #60a5fa;
+  }
+
+  .button:focus-visible {
+    outline-color: #60a5fa;
+  }
+}

src/app/error.tsx

@@ -0,0 +1,24 @@
+"use client";
+
+import style from "./error.module.css";
+
+interface ErrorProps {
+  error: Error & { digest?: string };
+  reset: () => void;
+}
+
+export default function ErrorPage({ error, reset }: ErrorProps) {
+  return (
+    <div className={style.container}>
+      <div className={style.card}>
+        <h1 className={style.title}>エラーが発生しました</h1>
+        <p className={style.description}>
+          予期せぬエラーが発生しました。しばらくしてからもう一度お試しください。
+        </p>
+        <button type="button" className={style.button} onClick={reset}>
+          再読み込み
+        </button>
+      </div>
+    </div>
+  );
+}

tsconfig.json

@@ -11,6 +11,7 @@
     "strict": true,
     "noEmit": true,
     "esModuleInterop": true,
+    "forceConsistentCasingInFileNames": true,
     "module": "esnext",
     "moduleResolution": "bundler",
     "resolveJsonModule": true,