fix(frontend): add skills API rewrite rule to prevent HTML fallback (#2241)

Fixes #2203

When NEXT_PUBLIC_BACKEND_BASE_URL is not set, the frontend uses Next.js
rewrites to proxy API calls to the gateway. Skills API routes were missing
from the rewrite config, causing /api/skills to return the SPA HTML instead
of JSON, which produced 'Unexpected token <' errors in the skill settings page.

Co-authored-by: JasonOA888 <JasonOA888@users.noreply.github.com>

frontend/next.config.js

@@ -52,6 +52,14 @@ const config = {
         source: "/api/agents/:path*",
         destination: `${gatewayURL}/api/agents/:path*`,
       });
+      rewrites.push({
+        source: "/api/skills",
+        destination: `${gatewayURL}/api/skills`,
+      });
+      rewrites.push({
+        source: "/api/skills/:path*",
+        destination: `${gatewayURL}/api/skills/:path*`,
+      });
     }
 
     return rewrites;