feat(merchants): scatter plot, merchant profiles, and per-merchant transaction history

- /merchants page with spend-vs-frequency scatter chart (click to open profile)
- Merchant profile drawer: stats, monthly trend line, full transaction history
- /api/analytics/merchants: split-adjusted merchant aggregates + monthly trends
- /api/analytics/merchants/[merchant]: per-merchant transaction list
- Add Merchants nav item to sidebar

src/lib/hooks.ts

@@ -557,3 +557,49 @@ export function useFees() {
     },
   });
 }
+
+export interface MerchantRow {
+  merchant: string;
+  category: string;
+  transaction_count: number;
+  total_spend: number;
+  avg_transaction: number;
+  first_seen: string;
+  last_seen: string;
+  months_active: number;
+  monthly_trend: Record<string, number>;
+}
+
+export function useMerchants(months = 12) {
+  return useQuery<{ merchants: MerchantRow[]; months: number }>({
+    queryKey: ["analytics", "merchants", months],
+    queryFn: async () => {
+      const res = await fetch(`/api/analytics/merchants?months=${months}`);
+      return res.json();
+    },
+  });
+}
+
+export interface MerchantTxnRow {
+  id: number;
+  transaction_date: string;
+  description: string;
+  amount: number;
+  amount_aud: number | null;
+  my_amount: number;
+  transaction_type: string;
+  category: string;
+  bank_name: string;
+  statement_id: number;
+}
+
+export function useMerchantTransactions(merchant: string | null) {
+  return useQuery<{ transactions: MerchantTxnRow[] }>({
+    queryKey: ["analytics", "merchant-txns", merchant],
+    queryFn: async () => {
+      const res = await fetch(`/api/analytics/merchants/${encodeURIComponent(merchant!)}`);
+      return res.json();
+    },
+    enabled: !!merchant,
+  });
+}