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-core/tests/e2e_mouse.rs

@@ -5,8 +5,7 @@
 
 mod common;
 use common::*;
-use ghostty_apprt::action::Action;
-use ghostty_apprt::structs::Clipboard;
+use ghostty_apprt::{action::Action, structs::Clipboard};
 use ghostty_input::{Key, MouseButton};
 
 const L: MouseButton = MouseButton::Left;