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/weak_ref.rs

@@ -2,8 +2,7 @@
 //! already provides `glib::WeakRef<T>`; this wrapper keeps the Zig API
 //! (`set`/`get`/`empty`).
 
-use gtk::glib;
-use gtk::glib::prelude::*;
+use gtk::{glib, glib::prelude::*};
 
 /// A weak reference to a GObject of type `T`.
 #[derive(Debug, Default)]