Use PNG instead of JPG for the payment QR image

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/routes/invoices/[id]/+page.svelte

@@ -52,7 +52,7 @@
 
   <section class="pay">
     <p class="muted">{$t('invoices.saved_thanks')}</p>
-    <img src="/payment-qr.jpg" alt="Payment QR code" class="qr" />
+    <img src="/payment-qr.png" alt="Payment QR code" class="qr" />
   </section>
 </article>