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>

main.cpp

@@ -3,7 +3,17 @@
 #include "core/panel_menu.h"
 #include "core/icons_tabler.h"
 #include "core/logger.h"
-// #include "viz/data_table.h"  // uncomment to enable data_table::render() panel
+#include "data_http.h"
+
+#include <string>
+#include <vector>
+
+// Config global del backend HTTP. Persistido por imgui.ini via Mantine-no-go.
+static std::string g_api_url = "http://127.0.0.1:8090";
+
+// Cache en memoria del primer fetch. Tabs proximos updates via WS.
+static std::vector<dag_ui::DagInfo> g_dags;
+static std::string                  g_last_error;
 
 // Toggles de paneles (visibles desde el menu View del menubar canonico)
 static bool g_show_main = true;
@@ -13,7 +23,25 @@ static void draw_main() {
         ImGui::End();
         return;
     }
-    ImGui::TextUnformatted("Hello from dag_engine_ui");
+    ImGui::Text("API: %s", g_api_url.c_str());
+    if (ImGui::Button("Fetch /api/dags")) {
+        g_dags.clear();
+        g_last_error.clear();
+        if (!dag_ui::list_dags_http(g_api_url, g_dags)) {
+            g_last_error = "list_dags_http failed (see stderr)";
+        }
+    }
+    if (!g_last_error.empty()) {
+        ImGui::TextColored(ImVec4(1, 0.4f, 0.4f, 1), "%s", g_last_error.c_str());
+    }
+    ImGui::Separator();
+    ImGui::Text("DAGs: %zu", g_dags.size());
+    for (auto& d : g_dags) {
+        ImGui::BulletText("%s  (%s)  schedule=%s",
+                          d.name.c_str(),
+                          d.valid ? "valid" : "invalid",
+                          d.schedule.empty() ? "none" : d.schedule[0].c_str());
+    }
     ImGui::End();
 }