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/app/api/transactions/route.ts

@@ -24,6 +24,7 @@ export async function GET(req: NextRequest) {
     offset: sp.get("offset") ? Number(sp.get("offset")) : undefined,
     amount_min: sp.get("amount_min") ? Number(sp.get("amount_min")) : undefined,
     amount_max: sp.get("amount_max") ? Number(sp.get("amount_max")) : undefined,
+    has_split: sp.get("has_split") || undefined,
   });
 
   return NextResponse.json(result);