fix(merchants): net spend accounting for refunds/credits

- Merchant totals now show net spend (gross debits minus refunds)
- Refund count and amount shown in profile drawer and table
- Scatter plot Y-axis uses net_spend, X-axis uses debit_count
- Per-merchant transaction history includes refunds (shown as negative)
- Monthly trend chart reflects net spend per month

src/lib/hooks.ts

@@ -562,9 +562,12 @@ export function useFees() {
 export interface MerchantRow {
   merchant: string;
   category: string;
-  transaction_count: number;
-  total_spend: number;
-  avg_transaction: number;
+  debit_count: number;
+  refund_count: number;
+  gross_spend: number;
+  total_refunds: number;
+  net_spend: number;
+  avg_debit: number;
   first_seen: string;
   last_seen: string;
   months_active: number;