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/SPRINT-PLAN.md

@@ -64,8 +64,9 @@ Notify user: **from Sprint 20 (Categories) once product↔category link + admin
 - [x] `docs/ADMIN.md` (new Sprint 26 section), `docs/BACKEND.md` item 15 added
 - Commit: `feat(admin): monitoring center`
 
-## Sprint 27 — Analytics
-- [ ] Dashboard extensions: sales/revenue/orders/visitors/products/categories charts, date ranges, export
+## Sprint 27 — Analytics ✅ done
+- [x] Sales/revenue/orders/products/categories (real aggregation over mock order data), visitors/funnels/heatmaps (pending-backend badges, no fabricated data), sales-over-time bar chart, 7/30/90-day ranges, CSV export
+- [x] `docs/ADMIN.md` (new Sprint 27 section), `docs/BACKEND.md` item 16 added
 - Commit: `feat(admin): analytics dashboard`
 
 ## Sprint 28 — Marketplace Polish