feat: add search to Transactions tab, spanning all history

Lets the user find a past purchase by keyword instead of paging through
months manually — search hits description/merchant/notes and ignores the
month filter, with a "jump to month" action on each result to view it in
its normal monthly context.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/api/transactions.ts

@@ -11,6 +11,7 @@ export interface TransactionFilters {
   account_id?: number;
   category_id?: number;
   type?: string;
+  search?: string;
   sort?: TransactionSort;
   page?: number;
   page_size?: number;