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/dashboard/templates/dashboard/home.html

@@ -32,6 +32,14 @@
     <div class="label">Open shop orders</div>
     <div class="value">{{ open_shop_orders|default:0 }}</div>
   </div>
+  <div class="stat-card">
+    <div class="label">Sales (30 days)</div>
+    <div class="value">{{ shop_sales_30d|default:0 }}</div>
+  </div>
+  <div class="stat-card">
+    <div class="label">Shop revenue (30 days)</div>
+    <div class="value" style="font-size:22px">{{ shop_revenue_30d|default:0 }}</div>
+  </div>
   <div class="stat-card">
     <div class="label">Low stock</div>
     <div class="value">{{ low_stock_products|default:0 }}</div>