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>

CMakeLists.txt

@@ -18,6 +18,7 @@ add_imgui_app(registry_dashboard
     main.cpp
     data.cpp
     data_http.cpp
+    http_client.cpp
     views.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/kpi_card.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/bar_chart.cpp
@@ -37,13 +38,7 @@ target_include_directories(registry_dashboard PRIVATE
 
 target_link_libraries(registry_dashboard PRIVATE SQLite::SQLite3)
 
-# cpp-httplib needs threads on Linux
-if(UNIX)
-    find_package(Threads REQUIRED)
-    target_link_libraries(registry_dashboard PRIVATE Threads::Threads)
-endif()
-
-# On Windows cross-compile, link ws2_32 for sockets
+# Sockets: ws2_32 on Windows, nothing extra on Linux
 if(WIN32)
     target_link_libraries(registry_dashboard PRIVATE ws2_32)
 endif()