Add optional signature upload to invoice PDF

Adds a drag-and-drop/click upload field on the invoice editor for an
optional 420x210 JPG/PNG signature image. The signature is stored as
part of invoice_data and rendered on the generated PDF directly above
the QR code, right-aligned with sensible margins.

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

app/test/app.test.js

@@ -30,4 +30,23 @@ describe('normalizeInvoiceData', () => {
             { mnozstvi: 'not-a-number', cenaZaMj: 10 }
         ] })).toBeNull();
     });
+
+    test('accepts a valid signature data URL and rejects invalid ones', () => {
+        const withValidSignature = normalizeInvoiceData({
+            items: [{ mnozstvi: 1, cenaZaMj: 10 }],
+            signature: 'data:image/png;base64,aGVsbG8='
+        });
+        expect(withValidSignature.signature).toBe('data:image/png;base64,aGVsbG8=');
+
+        const withInvalidSignature = normalizeInvoiceData({
+            items: [{ mnozstvi: 1, cenaZaMj: 10 }],
+            signature: 'not-a-data-url'
+        });
+        expect(withInvalidSignature.signature).toBeNull();
+
+        const withoutSignature = normalizeInvoiceData({
+            items: [{ mnozstvi: 1, cenaZaMj: 10 }]
+        });
+        expect(withoutSignature.signature).toBeNull();
+    });
 });