feat: receipt customization settings tab with header, footer, policy, social

- New Receipt tab in Settings page with editable fields
- receipt_header: text below logo (e.g. tagline)
- receipt_footer: thank you message
- receipt_return_policy: return policy text
- receipt_social: website/social media
- All stored in app_config, rendered on printed receipts
- Seeded in migration with empty defaults

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/admin/src/components/pos/pos-payment-dialog.tsx

@@ -63,6 +63,20 @@ export function POSPaymentDialog({ open, onOpenChange, paymentMethod, transactio
 
   const QUICK_AMOUNTS = [1, 5, 10, 20, 50, 100]
 
+  // Fetch receipt config
+  interface AppConfigEntry { key: string; value: string | null }
+  const { data: configData } = useQuery({
+    queryKey: ['config'],
+    queryFn: () => api.get<{ data: AppConfigEntry[] }>('/v1/config'),
+    enabled: !!result?.id,
+  })
+  const receiptConfig = {
+    header: configData?.data?.find((c) => c.key === 'receipt_header')?.value || undefined,
+    footer: configData?.data?.find((c) => c.key === 'receipt_footer')?.value || undefined,
+    returnPolicy: configData?.data?.find((c) => c.key === 'receipt_return_policy')?.value || undefined,
+    social: configData?.data?.find((c) => c.key === 'receipt_social')?.value || undefined,
+  }
+
   // Fetch full receipt data after completion
   const { data: receiptData } = useQuery({
     queryKey: ['pos', 'receipt', result?.id],
@@ -91,7 +105,7 @@ export function POSPaymentDialog({ open, onOpenChange, paymentMethod, transactio
               </Button>
             </div>
             <div className="print:block">
-              <POSReceipt data={receiptData} footerText="Thank you for your business!" />
+              <POSReceipt data={receiptData} config={receiptConfig} />
             </div>
           </DialogContent>
         </Dialog>