feat(insights): analytics drill-down, fee tracking, and category improvements

- Monthly spend chart with category breakdown drill-down
- Merchant frequency and spend analytics with per-merchant history
- Subscription detection and recurring charge tracking
- Fee and interest analytics endpoint
- Expanded category list with formatCategory display helper

src/app/api/merchants/route.ts

@@ -7,7 +7,7 @@ export async function GET(req: NextRequest) {
 
   if (type === "banks") {
     const banks = await getBankNames();
-    return NextResponse.json(banks.map((b) => b.bank_name));
+    return NextResponse.json(banks);
   }
 
   if (!search) return NextResponse.json([]);