fix: add missing Vercel rewrite rules for admin API routes

/admin/chat-history, /admin/proxies, /admin/dev/raw-samples,
and /admin/dev/captures were falling through to the SPA fallback
(/admin/index.html), causing "Unexpected token '<'" JSON parse
errors on the frontend.

vercel.json

@@ -81,6 +81,22 @@
       "source": "/admin/version",
       "destination": "/api/index"
     },
+    {
+      "source": "/admin/chat-history(.*)",
+      "destination": "/api/index"
+    },
+    {
+      "source": "/admin/proxies(.*)",
+      "destination": "/api/index"
+    },
+    {
+      "source": "/admin/dev/raw-samples/(.*)",
+      "destination": "/api/index"
+    },
+    {
+      "source": "/admin/dev/captures(.*)",
+      "destination": "/api/index"
+    },
     {
       "source": "/admin",
       "destination": "/admin/index.html"