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-os/src/string_encoding.rs

@@ -4,5 +4,8 @@
 //! vt cannot depend on os); this module re-exports it.
 
 pub use ghostty_vt::string_encoding::{
-    Error, printf_q_decode, url_percent_decode, url_percent_encode,
+    Error,
+    printf_q_decode,
+    url_percent_decode,
+    url_percent_encode,
 };