refactor: reemplazar cpp-httplib por HTTP client minimalista

cpp-httplib requiere std::mutex que no compila con MinGW win32
thread model. Se reemplaza por http_client.cpp: sockets crudos,
sin threading, sin SSL, funciona en ambos thread models.
Elimina vendor/httplib.h (10K lineas). nlohmann/json se mantiene.

main.cpp: doble clic sin argumentos intenta la API en localhost:8484
automaticamente. Si falla muestra pantalla de error con boton Retry.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

http_client.h

@@ -0,0 +1,30 @@
+#pragma once
+// Minimal HTTP client — no threading, no SSL, just plain TCP to localhost.
+// Works with both win32 and posix MinGW thread models.
+
+#include <string>
+
+struct HttpResponse {
+    int status = 0;
+    std::string body;
+    bool ok() const { return status >= 200 && status < 300; }
+};
+
+// Simple blocking HTTP GET/POST over TCP sockets.
+// host: "127.0.0.1", port: 8484
+class HttpClient {
+public:
+    HttpClient(const std::string& host, int port);
+
+    HttpResponse get(const std::string& path);
+    HttpResponse post(const std::string& path, const std::string& body,
+                      const std::string& content_type = "application/json");
+
+private:
+    std::string host_;
+    int port_;
+    int timeout_sec_ = 5;
+
+    HttpResponse request(const std::string& method, const std::string& path,
+                         const std::string& body, const std::string& content_type);
+};