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/css.rs

@@ -12,9 +12,7 @@
 
 use std::fmt::Write as _;
 
-use ghostty_config::config::Config;
-use ghostty_config::path::Path;
-use ghostty_config::types_b::WindowTheme;
+use ghostty_config::{config::Config, path::Path, types_b::WindowTheme};
 use gtk::{gdk, glib, prelude::*};
 
 /// Maximum size of a `gtk-custom-css` file.