feat(03-03): create error.vue (404 page) with i18n keys

- error.vue in app/ with statusCode display, i18n message, clearError redirect
- Added error.notFound, error.generic, error.backHome keys to fr.json and en.json

app/error.vue

@@ -0,0 +1,22 @@
+<script setup lang="ts">
+import type { NuxtError } from '#app'
+
+const props = defineProps<{ error: NuxtError }>()
+const { t } = useI18n()
+
+function handleError() {
+  clearError({ redirect: '/' })
+}
+</script>
+
+<template>
+  <div class="min-h-screen flex flex-col items-center justify-center gap-6 px-4">
+    <h1 class="text-8xl font-bold text-primary">{{ error.statusCode }}</h1>
+    <p class="text-xl text-gray-500 dark:text-gray-400 text-center max-w-md">
+      {{ error.statusCode === 404 ? t('error.notFound') : t('error.generic') }}
+    </p>
+    <UButton size="lg" @click="handleError">
+      {{ t('error.backHome') }}
+    </UButton>
+  </div>
+</template>