feat(i18n): add not-found page copy keys

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/i18n/locales/en-US.json

@@ -103,6 +103,9 @@
   "promo": "Limited offer, 50% off · Image-2 from $0.004/image for image generation and editing",
   "upgradeNow": "Upgrade now",
   "backHome": "Back home",
+  "notFoundTitle": "404: Page not found!",
+  "notFoundDescription": "We looked everywhere, but could not find the page you are looking for.",
+  "notFoundIllustration": "404 page not found",
   "save": "Save",
   "share": "Share",
   "shareLinkCopied": "Share link copied",

frontend/src/i18n/locales/zh-CN.json

@@ -103,6 +103,9 @@
   "promo": "限时优惠，立享5折 · Image-2 低至 $0.004/张，专注图片生成与编辑",
   "upgradeNow": "立即升级",
   "backHome": "返回首页",
+  "notFoundTitle": "404：页面未找到！",
+  "notFoundDescription": "我们到处都找过了，但找不到您要找的页面。",
+  "notFoundIllustration": "404 页面未找到",
   "save": "保存",
   "share": "分享",
   "shareLinkCopied": "分享链接已复制",