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-vt/src/kitty/color.rs

@@ -1,9 +1,9 @@
 //! Kitty color protocol (ZIG/src/terminal/kitty/color.zig).
 
-use crate::color::Rgb;
-use crate::osc::Terminator;
 use std::fmt;
 
+use crate::{color::Rgb, osc::Terminator};
+
 /// OSC 21 payload.
 #[derive(Debug, Clone, PartialEq, Default)]
 pub struct OSC {

crates/ghostty-vt/src/kitty/graphics_command.rs

@@ -69,8 +69,10 @@ fn parse_int(buf: &[u8], signed: bool) -> Result<u32, Error> {
 /// Decode base64 payload data. The reference implementation uses simdutf,
 /// which ignores ASCII whitespace and tolerates missing padding.
 pub fn decode_base64(input: &[u8]) -> Result<Vec<u8>, Error> {
-    use base64::Engine;
-    use base64::engine::{DecodePaddingMode, GeneralPurpose, GeneralPurposeConfig};
+    use base64::{
+        Engine,
+        engine::{DecodePaddingMode, GeneralPurpose, GeneralPurposeConfig},
+    };
     const ENGINE: GeneralPurpose = GeneralPurpose::new(
         &base64::alphabet::STANDARD,
         GeneralPurposeConfig::new()