feat(transactions): add imported date column, split filter, and sortable columns

- Show created_at as "Imported" column in transactions and shared views
- For reconciled transactions, show original CSV import date (not statement processing date) via LEFT JOIN on reconciled_with_id
- Add has_split filter (all/split only/unsplit only) to transactions page
- Transactions table: sortable by imported date; split filter dropdown
- Shared table: client-side sort by date, imported, and amount

src/lib/hooks.ts

@@ -26,6 +26,7 @@ interface TransactionFilters {
   offset?: number;
   amount_min?: number;
   amount_max?: number;
+  has_split?: string;
 }
 
 function buildParams(filters: TransactionFilters): string {