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

packages/ui/src/components/VBtn.vue

@@ -0,0 +1,30 @@
+<script setup lang="ts">
+const props = withDefaults(
+  defineProps<{
+    variant?: "default" | "primary" | "danger";
+    size?: "md" | "sm";
+    type?: "button" | "submit" | "reset";
+    disabled?: boolean;
+  }>(),
+  { variant: "default", size: "md", type: "button", disabled: false },
+);
+
+const base =
+  "inline-flex cursor-pointer items-center justify-center gap-1 rounded-md border text-sm font-medium transition-colors disabled:cursor-not-allowed disabled:opacity-50";
+const variants = {
+  default: "border-border bg-surface text-text hover:bg-bg",
+  primary: "border-primary bg-primary text-white hover:bg-primary-hover",
+  danger: "border-danger text-danger hover:bg-danger/10",
+} as const;
+const sizes = { md: "px-4 py-2", sm: "px-2.5 py-1 text-[13px]" } as const;
+</script>
+
+<template>
+  <button
+    :type="props.type"
+    :disabled="props.disabled"
+    :class="[base, variants[props.variant], sizes[props.size]]"
+  >
+    <slot />
+  </button>
+</template>