feat(frontend): add a dedicated 403 forbidden page

Show /403 instead of toasting or staying on the denied screen when
the API returns 403 or a non-admin opens an admin route.

frontend/proxy.ts

@@ -120,6 +120,7 @@ export function proxy(request: NextRequest) {
     '/privacy',
     '/terms',
     '/icon',
+    '/403',
   ];
   const publicPrefixes = ['/docs/', '/epay/'];