feat(sheets): status bar enhancements, prettier grid borders, consistent cell sizing
- Status bar now always visible: shows cell reference, selection dimensions, freeze state
- Click freeze indicator in status bar to unfreeze panes
- Grid uses border-separate with CSS custom properties (--color-grid-line, --color-grid-header-line)
- Borders between colored cells now render distinctly (no more disappearing borders)
- Cell height pinned to 26px with box-sizing: border-box for consistent sizing
- Hidden row indicators enlarged (4px → 6px) with hover tooltip "Click to unhide"
- Click hidden row indicator to unhide rows directly
- Validation error tooltips: hover invalid cells to see why (e.g., "Value must be one of: A, B, C")
- Dark mode grid line variables added
- Toolbar select and color inputs get focus-visible outlines