feat(admin): analytics dashboard

Sprint 27.

New features/admin/analytics/ module + net-new /:lang/backoffice/analytics
route + Dashboard Quick Action.

- revenue/orders/avg-order-value/sales-over-time/top-products computed by
  composing AdminOrdersLocalGateway (Sprint 23's seeded mock orders) - real
  aggregation over mock data, not a separate fabricated dataset
- products/categories counts from AdminProductsLocalGateway/
  AdminCategoriesLocalGateway
- visitors/funnels/heatmaps render pending-backend badges (no analytics
  pipeline exists anywhere in this system) rather than fabricated numbers,
  same convention as the Sprint 19 dashboard's pre-Sprint-23 Orders/Revenue
  cards
- plain div-bar chart (no charting library), 7/30/90-day range toggle,
  CSV export

docs/ADMIN.md + docs/BACKEND.md (new item 16) updated.

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

docs/ADMIN.md

@@ -391,6 +391,31 @@ routes at all, this is a net-new admin section.
   audit and Sprint 25's per-user audit. No consolidation attempted - they
   track different things.
 
+## Sprint 27 - Analytics
+
+`features/admin/analytics/`, net-new `/​:lang/backoffice/analytics` route
++ Dashboard Quick Action.
+
+- **Real, derived data**: revenue/orders/avg-order-value/sales-over-time
+  chart/top-products are computed by composing the existing
+  `AdminOrdersLocalGateway` (Sprint 23's seeded mock orders) - not a
+  separate fabricated dataset. Products/Categories counts come from
+  `AdminProductsLocalGateway`/`AdminCategoriesLocalGateway`. All of this is
+  still ultimately backed by mock order/product/category data (per those
+  sprints), but the *aggregation* is real arithmetic over that data, not
+  invented numbers.
+- **Visitors, funnels, heatmaps**: no analytics/tracking pipeline exists
+  anywhere in this codebase, so these render an explicit
+  "Awaiting backend integration" (`pending-backend`) badge, same convention
+  as the Sprint 19 dashboard's Orders/Revenue cards before Sprint 23 -
+  not fabricated numbers, not a generic empty state.
+- **Chart**: a plain inline `<div>`-bar chart driven by `[style.height.%]`,
+  no charting library pulled in - reasonable for one sales-over-time series
+  at this scale; revisit if more chart types are actually needed.
+- **Date ranges**: 7/30/90-day toggle filters orders by `createdAt`.
+- **Export**: CSV of the sales series (client-side `Blob` download, same
+  pattern as Orders/Transactions).
+
 ## Known gaps / backend needs
 
 - **Dashboard metrics endpoint.** Categories/Products counts are computed