pick
fun pickDocumentToOpen(app: AppModel, label: String, extensions: List<String>, callback: (String?) -> Unit)
Opens the browser's file picker through uapmd's Emscripten document provider, which creates a hidden <input type=file>, copies the chosen file into MEMFS and hands back a path the engine can open (DocumentProviderEmscripten.cpp:93).
extensions are matched the way the provider builds its accept string, e.g. ".uapmd". Returns null when the user cancels.