test(e2e): add 168 comprehensive Playwright tests across 14 spec files
Covers v0.5.0-v0.7.1 features: selection (Ctrl+A, Shift+Space, Ctrl+Space),
copy/paste toasts, status bar (cell ref, dimensions, freeze indicator,
click-to-unfreeze), undo/redo (keyboard + button + state), find & replace
(search, navigate, replace single/all), sheet tab management (rename,
duplicate, delete, move, tab color), freeze/hide via context menu + keyboard,
cell editor polish (placeholder, teal border, expansion), paste special
(values only, formatting only, transpose), formula error tooltips, validation
tooltips, toolbar state sync, keyboard shortcuts, visual CSS assertions
(border-collapse, background-clip, cell sizing, cursors), and hidden row
indicators with click-to-unhide.