feat(admin-web): add 404 page for unknown routes

Unknown URLs previously rendered a blank page. Add a catch-all route
under AppShell that shows the requested path plus back / workspace
actions, with zh-CN and en-US copy.

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

apps/admin-web/src/i18n/messages/en-US.ts

@@ -244,6 +244,18 @@ const enUS = {
       title: 'AI Point Usage',
       description: 'Review AI point balance, cycle rules, and recent usage records.',
     },
+    notFound: {
+      title: 'Page not found',
+      description: 'The link you opened might have moved or no longer exists.',
+    },
+  },
+  notFound: {
+    title: 'This page got lost',
+    description:
+      "We couldn't find anything at this address. The URL might be mistyped, or the page has been removed.",
+    pathLabel: 'Requested path',
+    back: 'Go back',
+    home: 'Back to workspace',
   },
   aiPoints: {
     balance: 'Available Points',