Remove X-App-Id header from SDK client

The X-App-Id header was a legacy mechanism for passing app identity to
the backend. App identity is now determined from URL path parameters in
all API routes, making this header redundant.

Part of base44-dev/gremlins#871

Author: Base44 Gremlin

src/client.ts

@@ -97,7 +97,6 @@ export function createClient(config: CreateClientConfig): Base44Client {
 
   const headers = {
     ...optionalHeaders,
-    "X-App-Id": String(appId),
   };
 
   const functionHeaders = functionsVersion