Refactor Nix handling and add toggle UI
- Introduce `NixModules` struct containing file paths, parsed ASTs, and
option lists. - Add `parse_nix` and `build_nix_modules` helpers for
centralized parsing. - Update CLI to use `build_nix_modules` instead of
manual parsing. - Extend `App` with system/home paths and ASTs, enabling
in‑place editing. - Implement boolean toggle on Enter: update AST,
rewrite file, refresh modules, and report action. - Simplify event
handling, adding shortcuts `1`/`2` to switch files. - Revise UI layout,
introduce footer with usage hints and last‑action status. - Adjust
imports and remove obsolete code across modules.
GarandPLG
6c50da8c18