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/specs/frontend-shop-admin/spec.md

@@ -2,9 +2,7 @@
 
 ## Purpose
 The merchant console for managing a shop's products and fulfilling its orders.
-
 ## Requirements
-
 ### Requirement: Merchant product management
 Shop users SHALL manage only their own shop's products: create/edit bilingual content, manage SKUs, publish/unpublish with immediate effect on the storefront.
 
@@ -51,3 +49,18 @@ Shop users SHALL manage only their shop's group-buying activities in shop-admin,
 #### Scenario: group buying appears in shop navigation
 - **WHEN** an authenticated shop user opens shop-admin
 - **THEN** a group-buying management entry is reachable without leaving the shop-scoped console
+
+### Requirement: Shared design system in the merchant console
+The shop-admin console SHALL use `@vmall/ui` primitives and `@vmall/shared` theme tokens for chrome (buttons, fields, cards, tables, page headers, nav). It MUST NOT depend on Element Plus or the removed `ui.css` stylesheet.
+
+#### Scenario: console chrome from the kit
+- **WHEN** an authenticated shop user opens shop-admin
+- **THEN** primary actions and page chrome render through the shared kit with the current accent preset
+
+### Requirement: Accent preset switcher
+Shop-admin SHALL expose the shared accent preset control (`red`, `blue`, `teal`, `violet`) in the authenticated shell header.
+
+#### Scenario: switcher visible
+- **WHEN** an authenticated shop user opens shop-admin
+- **THEN** they can select an accent preset without leaving the current page
+