Fix code input clearing behavior on download

Validate download code before starting download to ensure code input is only cleared on successful downloads. Invalid or expired codes now show an error message while preserving the entered code for correction.

src/templates/index.html

@@ -292,8 +292,26 @@
                 downloadButton.textContent = 'Downloading...';
 
                 try {
-                    window.location.assign(`/download/${code}`);
+                    // First check if the code is valid by making a HEAD request
-                    showMessage('Download started!', 'success');
+                    const checkResponse = await fetch(`/download/${code}`, {
+                        method: 'HEAD'
+                    });
+
+                    if (checkResponse.ok) {
+                        // Code is valid, start download and clear the input
+                        window.location.assign(`/download/${code}`);
+                        showMessage('Download started!', 'success');
+
+                        // Clear the code inputs after successful download
+                        setTimeout(() => {
+                            form.reset();
+                            downloadButton.disabled = true;
+                            deleteButton.disabled = true;
+                        }, 500);
+                    } else {
+                        // Code is invalid, show error and don't clear
+                        showMessage('Invalid or expired code. Please check and try again.', 'error');
+                    }
                 } catch (error) {
                     showMessage('Error starting download. Please try again.', 'error');
                 } finally {