design: comprehensive dark theme UI refresh (ui-ux-pro-max)
- index.css: new OLED-optimised CSS variables (rgba borders, layered
backgrounds, --ease/--duration tokens), global cursor:pointer on
buttons, focus-visible ring, input focus glow, .ac-card utility
- Sidebar: replace emoji icons with inline SVGs, add Dashboard/Upload
nav links with active states, + button for new sheet, better
sheet-item active treatment, polished context menu
- TopBar: SVG chevron back button, user avatar initials pill,
fixed 52px height, consistent spacing
- DashboardPage: hover lift on action cards, rgba-border sheet list,
dashed empty states, SVG icons
- SheetPage: sticky sub-header with saving indicator
- CommandBar: SVG mic + send icons, pill quick-starter chips,
rounded-lg inputs
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>