feat(ui): adopt Tailwind v4 design system and archive change

- Add tailwindcss v4 + @tailwindcss/vite to mall, shop-admin, admin
- Add @vmall/shared/theme.css tokens with html[data-accent] presets
- Add @vmall/ui kit (VBtn/VBadge/VField/VInput/VCard/VPanel/VTable/VPage,
  VAccentSwatch, useAccent) as a Nuxt module
- Convert all three apps to kit + utilities; delete ui.css/mall.css and
  every <style scoped>; consoles get accent presets, mall locked to red
- Fix VCard boolean prop default (padding) and PDP/store stale
  useAsyncData keys on param navigation
- Archive adopt-tailwind-design-system; new frontend-ui capability spec

openspec/changes/archive/2026-09-22-adopt-tailwind-design-system/proposal.md

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+## 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.