feat(mall): pin home category menu beside hero carousel

Home hero becomes a 450px flex row with a pinned 240x450 category rail
(ShellCategoryMenu `pinned` mode) left of the 960px center-cropped
carousel; hovering a top-level category expands the mega-menu over the
carousel. The header no longer collapses on scroll, and its category
dropdown is now hover-only on non-home pages.

OpenSpec change: openspec/changes/pin-home-category-menu

BREAKING CHANGE: the header logo/search/cart bar no longer auto-hides
past 200px of scroll, and the home page no longer shows the header
category dropdown (it is pinned in the hero row instead).

apps/mall/pages/index.vue

@@ -9,7 +9,10 @@ const floors = computed(() => homeFloors().filter((f) => f.products.length > 0))
 <template>
   <div>
     <div class="w1200 hero">
-      <UiCarousel :images="MOCK_BANNERS" :height="450" />
+      <ShellCategoryMenu pinned />
+      <div class="hero-slider">
+        <UiCarousel :images="MOCK_BANNERS" :height="450" />
+      </div>
     </div>
 
     <div class="w1200 strip">
@@ -49,7 +52,14 @@ const floors = computed(() => homeFloors().filter((f) => f.products.length > 0))
 
 <style scoped>
 .hero {
+  display: flex;
+  height: 450px;
   margin-top: 0;
+  background: #fff;
+}
+.hero-slider {
+  flex: 1;
+  min-width: 0;
 }
 .strip {
   display: flex;