#572: GTK: route gtk4::FileDialog calls through PlatformServices (Stage G of #493) - #586

…mServices

Msg::OpenFileDialog and Msg::SaveWorkspaceAsDialog now call
backend.services().show_file_open_dialog()/show_file_save_dialog()
(quadraui#427, now shipped) instead of constructing gtk4::FileDialog
in-line. dispatch() has no `backend` in scope, so the request is
stashed in a new PendingFileDialog Cell and drained in tick(), which
does receive the runner-owned `backend: &mut dyn quadraui::Backend` —
the same instance whose PlatformServices pump_depth is wired to the
runner's own event controllers, unlike App's separate self.backend
field (used for modal-stack/drag-state), which must NOT be used for
this (unshared pump_depth would reintroduce the #427 double-borrow
panic).

Msg::OpenFolderDialog stays on direct gtk4::FileDialog for now:
quadraui::PlatformServices has no folder-select dialog mode yet (only
file-open/file-save). That gap needs a new quadraui issue before the
third call site can migrate.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>