Extract file menu binding and Win32 splash helper

docs/modernization/roadmap.md

@@ -80,10 +80,10 @@ What is still carrying too much live ownership:
 Current hotspot files:
 
 - `src/canvas.cpp`: 2645 lines
-- `src/app_layout.cpp`: 1498 lines
+- `src/app_layout.cpp`: 1360 lines
 - `src/canvas_modes.cpp`: 1798 lines
 - `src/node.cpp`: 1551 lines
-- `src/main.cpp`: 1374 lines
+- `src/main.cpp`: 1181 lines
 - `src/node_panel_brush.cpp`: 1197 lines
 - `src/node_stroke_preview.cpp`: 933 lines
 - `src/node_canvas.cpp`: 888 lines
@@ -127,7 +127,9 @@ Current architecture mismatches that must be treated as real blockers:
   rather than thin composition/binding surfaces, even though tools-menu binding
   plus nested panels/options submenu wiring now live in
   `src/app_layout_tools_menu.cpp` and `App::init_menu_tools()` is now a thin
-  call-through.
+  call-through, while file-menu binding plus the export submenu wiring now also
+  live in `src/app_layout_file_menu.cpp` and `App::init_menu_file()` is now a
+  thin call-through.
 - `App`, `Canvas`, `Node`, retained workers, and platform entrypoints still use
   global singleton reach, raw observer pointers, retained static worker
   ownership in several app families, and ad hoc mutex/condition-variable
@@ -149,7 +151,10 @@ Current architecture mismatches that must be treated as real blockers:
   local worker-state helper instead of a bare static accessor, the
   prepared-file worker and the canvas async import/export/save/open worker now
   live under `AppRuntime` instead of retained static app-events/canvas
-  workers, `App::rec_loop()` now delegates worker-iteration orchestration into
+  workers, and the splash-screen dialog loop, HWND ownership, and bitmap setup
+  now live in `src/platform_windows/windows_splash.cpp` instead of
+  `src/main.cpp`,
+  while `App::rec_loop()` now delegates worker-iteration orchestration into
   the retained recording bridge, `App::update_rec_frames()` now delegates
   recording label refresh through that same retained recording path, and the
   canvas state-management cluster for picking, clear/clear-all, layer

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