Fix IME composition key routing - Korean support

[dfdgsdfg]

Korean Support

Summary

• Added an ime_state helper module for IME key-event routing.
• Fixed composition commits so IME text is sent through Ghostty key events directly,
  while normal key commits still keep physical key metadata.
• Added GTK IM context focus in/out handling and wired the new IME state tests into zig build test.

Validation

• git diff --check passed.
• zig build test was not run because zig is not installed in this environment.

[dfdgsdfg]

Tested in an actual GNOME environment.

• Confirmed that Korean/CJK input works correctly.
• Also tested entering CJK text after moving the cursor into the middle of an existing sentence.

This looks good to merge now.