Design the target UI first (Storybook prototype): foundations and key screens, before implementing the modernisation

Part of PAR-357. Do this before Phase 2 work starts.

Why

The current UI grew screen by screen in code, which is how it ended up with 8 font sizes, glyphs standing in for icons, and every element in a bordered box. The modernisation touches every surface. Implementing ~40 tasks without a visual target will reproduce the same inconsistency in a new style.

Do

1. Foundations page: brand palette (PAR-358), the type scale and roles (PAR-359), spacing, radii, elevation (PAR-395), the icon set (PAR-393), illustration style (empty states task). Light and warm-dark.
2. Components: buttons (primary/secondary/ghost/destructive × sizes), inputs, pills and chips, status/priority/label icons, avatar, menus, popover, date picker, toast, dialog, sheet, command palette, tooltip, list row, board card, sidebar item, page header and toolbar.
3. Key screens (desktop plus mobile):
   • issues list
   • board
   • issue detail with sidebar
   • quick-create modal
   • filters and display popovers
   • command palette
   • inbox
   • project overview
   • releases / changelog
   • settings
   • sign-in and onboarding
   • the public-workspace guest view
   • empty states
4. Validate against the Mobbin references in the epic and against the landing mock (it should read as the same product).
5. Hand-off: map the Figma components to @tasklie/design-system (Code Connect) and attach frame links to each implementation task.

Acceptance

• An approved Figma file with foundations, components and the screens above.
• Every Phase 2–4 task links to its frame.