style: apply nightly rustfmt import grouping across the workspace

rustfmt.toml sets group_imports, imports_granularity and imports_layout,
which are nightly-only; agents formatted with stable rustfmt, which
ignored them. This is a formatting-only pass with `cargo +nightly fmt`.

Test Plan:
- cargo clippy --workspace --all-targets -- -D warnings
- cargo test --workspace

crates/ghostty-apprt-gtk/src/class/surface_child_exited.rs

@@ -6,11 +6,9 @@
 
 use std::sync::OnceLock;
 
-use adw::prelude::*;
-use adw::subclass::prelude::*;
+use adw::{prelude::*, subclass::prelude::*};
 use ghostty_apprt::surface::ChildExited;
-use gtk::glib;
-use gtk::glib::subclass::Signal;
+use gtk::{glib, glib::subclass::Signal};
 
 /// Boxed wrapper so `ChildExited` can be a GObject property.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, glib::Boxed)]