Add a shop sales dashboard with 30-day charts (#8)

## Summary
- Closes #7
- Portal Retail **Sales** page: 30-day orders/revenue/units/AOV, daily charts, top products
- Home dashboard cards for 30-day shop sales and revenue when `FEATURE_SHOP` is on
- Stacks on #6 (shop catalog). Merge that first, then retarget `master` if needed.

## Test plan
- [ ] Sales page requires login; empty state with no paid orders
- [ ] Paid/fulfilled in the last 30 days counted; draft/open/cancelled/old excluded
- [ ] Top products ranked by units; home dashboard shows 30-day sales count
- [ ] `manage.py test shop core.tests_features`

Reviewed-on: #8

site/public/context_processors.py

@@ -63,6 +63,15 @@ def tianji_tracking(request):
         nav_section = "social_accounts"
     elif namespace == "social":
         nav_section = "social"
+    elif namespace == "shop_portal":
+        if url_name.startswith("product"):
+            nav_section = "shop_products"
+        elif url_name.startswith("order"):
+            nav_section = "shop_orders"
+        elif url_name == "sales":
+            nav_section = "shop_sales"
+    elif namespace == "shop":
+        nav_section = "shop"
 
     return {
         "tianji_enabled": getattr(settings, "TIANJI_ENABLED", False)