feat(ui): add native multi-window application support

- let UIApplication create, manage, render, and destroy multiple native windows
  - add scoped scene and GL window-context switching
  - route input events to the scene belonging to each native window
  - share application fonts, themes, icons, styles, and graphics resources safely
  - add NewInApplicationWindow factories for UIWindow, UIFileDialog, and UIMessageBox
  - support native parent relationships, modality, minimum sizes, and optional positioning
  - close child windows when the primary application window closes
  - defer native context destruction until the end of the application tick
  - fall back to in-scene windows for Emscripten and terminal rendering
  - reapply renderer state caches when switching OpenGL contexts
  - fix SDL2/SDL3 display bounds and multi-window lifecycle handling
  - add a native file-dialog and message-box multi-window example
  - add integration coverage for context switching, input, resources, modality, and closing

.ecode/project_build.json

@@ -411,6 +411,12 @@
         "command": "${project_root}/bin/eepp-ui-data-handling-debug",
         "name": "eepp-ui-data-handling-debug",
         "working_dir": "${project_root}/bin"
+      },
+      {
+        "args": "",
+        "command": "${project_root}/bin/eepp-ui-application-multi-window-debug",
+        "name": "eepp-ui-application-multi-window-debug",
+        "working_dir": "${project_root}/bin"
       }
     ],
     "var": {