Feature: disable preview button until CSV file is selected

Added validation to prevent users from submitting the upload form without
selecting a file first. The preview button is now disabled by default and
only enables when a file is selected, improving user experience and
preventing empty form submissions.

Changes:
- Added IDs to file input and preview button
- Button starts in disabled state
- JavaScript listener enables button when file is selected
- Button disables again if file selection is cleared

MoneyMap/wwwroot/js/upload.js

@@ -113,4 +113,14 @@ document.addEventListener('DOMContentLoaded', function() {
     if (totalRows > pageSize) {
         showPage(0);
     }
+
+    // Enable/disable preview button based on file selection
+    const csvFileInput = document.getElementById('csvFileInput');
+    const previewButton = document.getElementById('previewButton');
+
+    if (csvFileInput && previewButton) {
+        csvFileInput.addEventListener('change', function() {
+            previewButton.disabled = !this.files || this.files.length === 0;
+        });
+    }
 });