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-config/src/file_formatter.rs

@@ -1,11 +1,14 @@
 //! `FileFormatter` (part of `config/formatter.zig`): outputs the config in a
 //! file-like format, identical to Ghostty's `+show-config`.
 
-use crate::config::Config;
-use crate::key::Key;
-use crate::macros::{ConfigStruct, help_string};
 use std::fmt;
 
+use crate::{
+    config::Config,
+    key::Key,
+    macros::{ConfigStruct, help_string},
+};
+
 /// FileFormatter is a formatter implementation that outputs the
 /// config in a file-like format. This uses more generous whitespace,
 /// can include comments, etc.