fix(storefront): release-candidate walkthrough fixes

- Cart item description showed a stray literal "..." when the item had no
  description text (cart.component.html) — now only renders the trailing
  ellipsis when a description is present.
- Compare table showed raw internal stock enum values ("high"/"low"/etc.)
  instead of localized labels (compare-table.component.ts) — now reuses the
  same stock-label mapping used by product cards.
- Search with zero results incorrectly showed the empty-category messaging
  ("browse categories" / "go to parent category") stacked on top of the
  search's own "nothing found" message (catalog-container.component.ts) —
  isEmptyCategoryState now excludes active search queries so only the
  search-appropriate empty state renders.
- Footer "About" link pointed to /about, which 404s; the actual CMS page
  route is /about-us (bootstrap.json mock nav data) — corrected the route.
- Added missing public/assets/images/placeholder.svg, the fallback image
  referenced by getMainImage() for items without photos (previously 404s
  if that fallback path is ever hit).

Investigated and left as-is (not code bugs): /images/*.webp 404s on
product cards are references to a real backend/CDN not present in local
dev (confirmed via mock-data.interceptor.ts and api.service.ts image-URL
resolution) — expected dev-only gap. Footer "Contacts" link (/contacts)
has no corresponding static page content at all in mock data; flagging
for a content decision rather than fabricating copy.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

public/assets/images/placeholder.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 400 300" role="img" aria-label="No image available">
+  <rect width="400" height="300" fill="#e5e7eb"/>
+  <g fill="none" stroke="#9ca3af" stroke-width="2">
+    <rect x="40" y="40" width="320" height="220" rx="8"/>
+    <path d="M40 220 L140 130 L200 180 L260 110 L360 220" />
+    <circle cx="140" cy="100" r="20"/>
+  </g>
+</svg>