Change the style of the PDF export button to match other elements

Update className of the PDF export button in SalesHistory.tsx to `modern-button-secondary` for visual consistency.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: dec54137-d5a2-4406-8ad4-5f2f9fb110bf
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/0715008c-7244-40f4-befc-26c014852236/21a454be-413a-4934-8745-041d5f8dbc78.jpg

client/src/components/SalesHistory.tsx

@@ -332,7 +332,7 @@ export default function SalesHistory({ canDelete = false }: SalesHistoryProps) {
               <button
                 onClick={exportToPDF}
                 disabled={filteredSales.length === 0}
-                className="modern-button modern-button-primary"
+                className="modern-button modern-button-secondary"
               >
                 <FileText className="h-4 w-4" />
                 Export PDF