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>

data_http.h

@@ -0,0 +1,84 @@
+#pragma once
+
+// Cliente HTTP REST contra apps/dag_engine (Go backend).
+// Endpoints: /api/dags, /api/runs, /api/dags/{name}/run (issue 0095).
+
+#include <string>
+#include <vector>
+
+namespace dag_ui {
+
+// --- Modelo de datos (mirror JSON shape de apps/dag_engine) ---
+
+struct DagInfo {
+    std::string              name;
+    std::string              description;
+    std::vector<std::string> schedule;     // cron expressions
+    std::vector<std::string> tags;
+    std::string              type;
+    std::string              file_path;
+    bool                     valid = false;
+    // last_run inlined fields (extracted from nested object on load)
+    bool                     has_last_run = false;
+    std::string              last_run_id;
+    std::string              last_run_status;
+    std::string              last_run_started_at;
+    std::string              last_run_finished_at;
+};
+
+struct DagRunRow {
+    std::string id;
+    std::string dag_name;
+    std::string dag_path;
+    std::string status;        // pending|running|success|failed|cancelled
+    std::string trigger;       // manual|cron|api
+    std::string started_at;
+    std::string finished_at;
+    std::string error;
+};
+
+struct DagStepRow {
+    std::string id;
+    std::string run_id;
+    std::string step_name;
+    std::string status;
+    int         exit_code = 0;
+    std::string stdout_text;
+    std::string stderr_text;
+    std::string started_at;
+    std::string finished_at;
+    long long   duration_ms = 0;
+    std::string error;
+};
+
+struct DagRunDetail {
+    DagRunRow              run;
+    std::vector<DagStepRow> steps;
+};
+
+struct DagDetail {
+    DagInfo                 info;
+    std::vector<DagRunRow>  recent_runs;
+    std::string             validation_error;     // empty if valid
+};
+
+// --- API ---
+// api_url: "http://127.0.0.1:8090". Todas las funciones blocking, devuelven
+// false si la red falla o el status no es 2xx. Errores van a stderr.
+
+bool list_dags_http(const std::string& api_url, std::vector<DagInfo>& out);
+
+bool get_dag_http(const std::string& api_url, const std::string& name,
+                  DagDetail& out);
+
+bool list_runs_http(const std::string& api_url, const std::string& dag_name,
+                    int limit, std::vector<DagRunRow>& out);
+
+bool get_run_http(const std::string& api_url, const std::string& run_id,
+                  DagRunDetail& out);
+
+// Devuelve el run_id en out_run_id si HTTP 202.
+bool trigger_dag_http(const std::string& api_url, const std::string& name,
+                      std::string& out_run_id, std::string& out_error);
+
+} // namespace dag_ui