feat(mall): read the store directory from the API

Wave 5: the store directory, store home and product-page store card stop reading
MOCK_STORES, and the payment and order surfaces name their shop.

- a `shop_profiles` table beside `shops`, so the identity model both consoles
  consume is untouched, with a public `GET /api/shops` and `GET /api/shops/{slug}`
  and an admin `PUT /api/admin/shops/{id}/profile`
- a shop with no profile is still listed, with the fields absent rather than
  invented; the pages guard every block, and a missing logo renders an
  initial-letter placeholder
- `scripts/seed-demo.mjs` upserts a profile per demo shop, since profiles hang
  off shops that script creates
- payment and order pages resolve shop ids to names from one cached shop read,
  retiring the generic "Shop" label
- three things went rather than being faked, following the wave-1 precedent:
  `distanceKm` and its sort (no geo model), the store home's sales/comments
  sorts, and its "best sellers" rail (no sales model)
- `lowestSku` moved out of the fixed-data module into `apps/mall/utils/product.ts`
  and re-exported, so live pages stop importing the mock module for a pure
  helper

Verified: 28 backend tests green including five new shop tests; all three
frontends build; the directory, store home, store card and order cards all render
real data with no distance or sales claims; the fixed-data rollback still renders
the store surfaces with the backend stopped.

Note: `nuxt build` does not typecheck in this repo (no `typescript.typeCheck`,
no `vue-tsc`), which AGENTS.md implies it does. A re-export used here created no
local binding and broke internal callers at runtime while the build stayed green;
`docs/TBD-migrate-wave.md` records the gap.

OpenSpec change: openspec/changes/replace-mock-api-wave-5

apps/mall/mock/data.ts

@@ -491,15 +491,10 @@ export function storeById(idOrSlug: string): MockStore | null {
   return MOCK_STORES.find((s) => s.id === idOrSlug || s.slug === idOrSlug) ?? null;
 }
 
-export function lowestSku(product: Product): Sku | null {
-  const active = (product.skus ?? []).filter((s) => s.active && s.stock > 0);
-  return (
-    active.reduce<Sku | null>(
-      (low, s) => (low === null || s.price_minor < low.price_minor ? s : low),
-      null,
-    ) ?? (product.skus ?? []).find((s) => s.active) ?? null
-  );
-}
+import { lowestSku } from "~/utils/product";
+
+/** Kept for the mock-era pages; live pages import it from `~/utils/product`. */
+export { lowestSku };
 
 export interface MockSearchQuery {
   q?: string;