feat(sheets): bold/italic button state sync, Ctrl+A select all, expanded tests
- Bold and italic toolbar buttons now reflect current cell state when navigating
- Ctrl+A / Cmd+A selects all cells in the sheet
- moveSelectionTo also updates all toolbar button states (bold, italic, underline, strikethrough, font size/family)
- Added 7 new tests: select all, status bar info, freeze indicator text, toolbar state sync