refactor(shaders_lab): usar modal_dialog en save-as (issue 0046)

El modal Save-as-generator usaba BeginPopupModal + InputText + Button
crudo. Ahora usa fn_ui::modal_dialog_begin/end + fn_ui::text_input +
fn_ui::button del registry. El error inline usa fn_tokens::colors::error
en vez de ImVec4(1, 0.4, 0.4, 1). Anade modal_dialog.cpp, text_input.cpp
y button.cpp al CMakeLists del app.

Raw ImGui::Begin*/Selectable/BeginPopupModal: 11 -> 8.

cpp/apps/shaders_lab/CMakeLists.txt

@@ -17,6 +17,10 @@ add_imgui_app(shaders_lab
     ${CMAKE_SOURCE_DIR}/functions/gfx/dag_node_previews.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/shaderlab_db.cpp
     ${CMAKE_SOURCE_DIR}/functions/gfx/code_to_generator.cpp
+    # Primitivos UI usados por el modal Save-as-generator.
+    ${CMAKE_SOURCE_DIR}/functions/core/modal_dialog.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/text_input.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/button.cpp
     # fps_overlay, panel_menu, layouts_menu, app_menubar, layout_storage ya
     # viven en fn_framework.
 )