refactor(shaders_lab): migrar layouts inline a layout_storage publico

Sustituye ~30 lineas de cableado manual de save/load/list/delete contra
layout_storage_sqlite por dos llamadas a la nueva API publica:

    g_layouts = fn_ui::layout_storage_open("shaders_lab.db");
    fn_ui::layout_storage_make_callbacks(g_layouts, g_layout_cb);

El blob pendiente lo gestiona el storage (layout_storage_apply_pending).
on_reset se override para ademas re-mostrar los paneles de shaders_lab.
La tabla ui_layouts heredada queda intacta — la nueva API usa
imgui_layouts en la misma BD.

cpp/apps/shaders_lab/CMakeLists.txt

@@ -16,8 +16,8 @@ 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
-    # fps_overlay, panel_menu, layouts_menu, app_menubar ya viven en fn_framework
-    ${CMAKE_SOURCE_DIR}/functions/core/layout_storage_sqlite.cpp
+    # fps_overlay, panel_menu, layouts_menu, app_menubar, layout_storage ya
+    # viven en fn_framework.
 )
 target_include_directories(shaders_lab PRIVATE
     ${CMAKE_CURRENT_SOURCE_DIR}