## Why

Styling is split across `ui.css`, `mall.css`, and ~4k lines of Vue scoped CSS, with two palettes (mall red vs admin blue) and no shared primitives. Adopting Tailwind v4 plus one token/kit package lets us rewrite every surface, refresh visuals, and give consoles accent presets without a second CSS stack.

## What Changes

- **BREAKING**: Remove `@vmall/shared/ui.css` and `apps/mall/assets/mall.css`. Shared contract export becomes `theme.css`.
- Add Tailwind v4 (`@tailwindcss/vite`) to mall, shop-admin, and platform admin.
- Add `@vmall/ui`: Vue primitives (`VBtn`, `VBadge`, `VField`/`VInput`, `VCard`/`VPanel`, `VTable`, `VPage`) plus `useAccent()`.
- One design system: commerce red (`#ca151e`) default; same radius/type/spacing on all three apps.
- Mall keeps classic PC chrome (1200px grid, dark nav, mega-menu, 450px home hero) with refreshed tokens.
- Shop-admin and platform admin: accent presets `red | blue | teal | violet`, persisted in `localStorage`. Mall stays red with no switcher.
- Delete all Vue `<style scoped>` in the three apps after conversion.

## Capabilities

### New Capabilities

- `frontend-ui`: Shared Tailwind theme, UI kit, and console accent presets.

### Modified Capabilities

- `frontend-mall`: Storefront MUST use the shared design system; visual IA (1200px / `#ca151e` / hero / mega-menu) stays; no public accent switcher.
- `frontend-admin`: Platform console MUST use the shared kit and expose accent presets.
- `frontend-shop-admin`: Merchant console MUST use the shared kit and expose accent presets.

## Impact

`packages/shared` (`ui.css` → `theme.css`), new `packages/ui`, three Nuxt apps (all SFCs), `AGENTS.md` / OpenSpec config (shared styles path). No API or money-handling changes. Three-app `pnpm` builds plus a mall visual pass.

## Non-goals

Dark mode, free hex picker, per-shop customer-facing branding, theming the public mall, Element Plus, mixing this with the catalog-module split.