Extract file menu binding and Win32 splash helper

docs/modernization/tasks.md

@@ -271,7 +271,7 @@ targets look like helpers under one old monolith.
 Status: In Progress
 
 Why now:
-`src/app_layout.cpp` is still a 1498-line mixed file that builds menus,
+`src/app_layout.cpp` is still a 1360-line mixed file that builds menus,
 attaches callbacks, computes planner inputs, and mutates UI state directly.
 
 Current slice:
@@ -279,6 +279,10 @@ Current slice:
   now lives in `src/app_layout_tools_menu.cpp`, and `App::init_menu_tools()`
   is now a thin call-through, but file/about/layer/sidebar and broader layout
   composition are still inline in `src/app_layout.cpp`.
+- File-menu UI binding, including the export submenu wiring, now also lives in
+  `src/app_layout_file_menu.cpp`, and `App::init_menu_file()` is now a thin
+  call-through, but about/layer/sidebar and broader layout composition are
+  still inline in `src/app_layout.cpp`.
 
 Write scope:
 - `src/app_layout.cpp`
@@ -392,6 +396,9 @@ Current slice:
 - `main.cpp` Win32 window handles, GL task/mutex state, splash-dialog state,
   stylus timers, and VR worker state now sit behind one retained local state
   object instead of separate file-scope globals
+- the splash-screen dialog loop, HWND ownership, and bitmap setup now also live
+  in `src/platform_windows/windows_splash.cpp` instead of `src/main.cpp`, and
+  `main.cpp` now just owns the helper lifecycle
 - Win32 async GL/context lock state now lives in
   `src/platform_windows/windows_platform_services.cpp` instead of `main.cpp`
   retained state, and `main.cpp` only seeds that platform-owned context handle