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>

CMakeLists.txt

@@ -1,5 +1,7 @@
 add_imgui_app(dag_engine_ui
     main.cpp
+    http_client.cpp
+    data_http.cpp
 )
 target_include_directories(dag_engine_ui PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})