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/cgroup.rs

@@ -1,8 +1,6 @@
 //! Transient systemd scope creation over D-Bus (port of `apprt/gtk/cgroup.zig`).
 
-use gtk::gio;
-use gtk::glib;
-use gtk::prelude::*;
+use gtk::{gio, glib, prelude::*};
 
 /// Properties for the transient scope.
 #[derive(Debug, Clone, Copy, Default)]