On Windows, native terminals don't support POSIX suspend (ctrl+z), so we now
assign ctrl+z to input undo instead of terminal suspend. Terminal suspend is
disabled on Windows to avoid conflicts with the undo functionality.
Extract error handling, parsing logic, and variable substitution into dedicated
modules. This reduces duplication between tui.json and opencode.json parsing
and makes the config system easier to extend for future config formats.
Dax