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/lib/categories.ts

@@ -1,3 +1,9 @@
+export const REGULAR_CATEGORIES = new Set([
+  "rent", "utilities", "insurance", "subscriptions",
+  "groceries", "dining", "transport", "fuel",
+  "health", "personal_care", "government", "charity", "pets",
+] as const);
+
 export const CATEGORIES = [
   "groceries",
   "dining",