Add saved signature presets

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

app/test/app.test.js

@@ -1,4 +1,4 @@
-const { normalizeInvoiceData } = require('../src/invoice-data');
+const { isValidSignatureDataUrl, normalizeInvoiceData } = require('../src/invoice-data');
 
 describe('normalizeInvoiceData', () => {
     test('calculates the total from validated line items', () => {
@@ -49,4 +49,10 @@ describe('normalizeInvoiceData', () => {
         });
         expect(withoutSignature.signature).toBeNull();
     });
+
+    test('validates only supported base64 signature image data URLs', () => {
+        expect(isValidSignatureDataUrl('data:image/jpeg;base64,aGVsbG8=')).toBe(true);
+        expect(isValidSignatureDataUrl('data:image/gif;base64,aGVsbG8=')).toBe(false);
+        expect(isValidSignatureDataUrl('data:image/png;base64,not valid')).toBe(false);
+    });
 });