feat: add client support for proxy authentication

- Add MCP_PROXY_AUTH_TOKEN configuration field to store session token
- Update useConnection hook to include proxy auth token in all requests
- Add separate proxy auth headers to avoid conflicts with MCP server auth
- Add specific error handling for proxy authentication failures
- Show helpful toast message directing users to Configuration settings
- Update README with instructions for configuring token in UI

The client now properly authenticates with the proxy server using the
session token displayed in the proxy console.

client/src/lib/constants.ts

@@ -54,4 +54,10 @@ export const DEFAULT_INSPECTOR_CONFIG: InspectorConfig = {
       "Set this if you are running the MCP Inspector Proxy on a non-default address. Example: http://10.1.1.22:5577",
     value: "",
   },
+  MCP_PROXY_AUTH_TOKEN: {
+    label: "Proxy Session Token",
+    description:
+      "Session token for authenticating with the MCP Proxy Server (displayed in proxy console on startup)",
+    value: "",
+  },
 } as const;