Implements the App side of T5 (toast notifications from the bottom/tlc
synthesis). render_toast() overlay can be wired in a follow-up pass.
- App.toast: Option<String> field
- App.toast_expires: Option<Instant> field
- App.flash_toast(msg): sets toast with 3s auto-dismiss
- App.active_toast(): returns &str if not expired
Key bindings: scrollable help dialog (j/k/PgUp/PgDn/Home/G) wired in
the previous round. dim_backdrop/render_nice_edit/render_open_files
helpers lost to repeated reverts — will be re-added in a clean follow-up.
vasilito