fix: use X-MCP-Proxy-Auth header for proxy authentication

Changed proxy authentication to use a custom header (X-MCP-Proxy-Auth) instead of the standard Authorization header. This prevents conflicts when the proxy needs to forward the client's Authorization header to upstream MCP servers.

- Updated client to send proxy auth token in X-MCP-Proxy-Auth header
- Updated server auth middleware to check X-MCP-Proxy-Auth header
- Applies to all transport types (SSE, stdio, streamable-http)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

client/src/lib/hooks/useConnection.ts

@@ -246,7 +246,7 @@ export function useConnection({
       const proxyAuthToken = getMCPProxyAuthToken(config);
       const headers: HeadersInit = {};
       if (proxyAuthToken) {
-        headers["Authorization"] = `Bearer ${proxyAuthToken}`;
+        headers["X-MCP-Proxy-Auth"] = `Bearer ${proxyAuthToken}`;
       }
       const proxyHealthResponse = await fetch(proxyHealthUrl, { headers });
       const proxyHealth = await proxyHealthResponse.json();
@@ -335,7 +335,7 @@ export function useConnection({
       const proxyAuthToken = getMCPProxyAuthToken(config);
       const proxyHeaders: HeadersInit = {};
       if (proxyAuthToken) {
-        proxyHeaders["Authorization"] = `Bearer ${proxyAuthToken}`;
+        proxyHeaders["X-MCP-Proxy-Auth"] = `Bearer ${proxyAuthToken}`;
       }
 
       // Create appropriate transport