feat: cliente HTTP REST + main panel demo (issue 0095 step 3)

- http_client.{h,cpp}: TCP plain (copia de registry_dashboard, sin SSL).
- data_http.{h,cpp}: API DAG (list/get/runs/trigger) + parser nlohmann/json.
- vendor/nlohmann/json.hpp: vendored.
- main.cpp: panel "Fetch /api/dags" demo, lista DAGs con schedule.
- CMakeLists.txt: anade http_client.cpp + data_http.cpp.

Build verificado. WS y tabs (data_table::render) en commits siguientes.

Co-Authored-By: Claude Opus 4.7 (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);
+};