Add a shop sales dashboard with 30-day charts (#4)

## Summary
- Closes #3
- Portal Retail **Sales** page: 30-day orders/revenue/units/AOV, daily charts, top products
- Home dashboard cards for 30-day shop sales and revenue
- Stacks on #2. 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; unpaid/old excluded
- [ ] Home dashboard shows 30-day sales count
- [ ] `manage.py test shop`

Reviewed-on: #4

site/core/tests_features.py

@@ -123,6 +123,7 @@ class InstalledOptionalAppsTests(TestCase):
         self.assertTrue(reverse("social_ai:generate").startswith("/portal/social/api/generate/"))
         self.assertTrue(reverse("shop:list").startswith("/shop/"))
         self.assertTrue(reverse("shop_portal:product_list").startswith("/portal/shop/"))
+        self.assertTrue(reverse("shop_portal:sales").startswith("/portal/shop/sales/"))
         self.assertTrue(reverse("pos_sync:event_list").startswith("/portal/pos/"))
         self.assertTrue(reverse("events:list").startswith("/events/"))
         self.assertTrue(reverse("events_portal:event_list").startswith("/portal/events/"))