TXT Sheets v0.2.8

Fixes file closing and duplicate workbook windows, recovery cleanup, and saving unfinished cell edits.

Fixes large-sheet MIN/MAX calculations and CSV export. Row and column insertions/deletions now update formula references throughout the workbook, including ranges, cross-sheet references, and deleted-reference errors. Undo/redo and saved Excel recalculation are preserved.

Validation: 56 automated tests, production build, and Electron save/close, multi-window, keyboard, sheet-order, structural editing, and save/reopen checks.