fix(frontend): lower QR code error correction level to reduce density

Payment QR codes used 'H' (30% redundancy) error correction which made
the codes too dense to scan on some phones. Lower to 'M' (15%) when a
logo overlay is present and 'L' (7%) without logo, significantly
reducing module density while maintaining scannability.

Closes #1607

frontend/src/components/payment/PaymentQRDialog.vue

@@ -154,7 +154,7 @@ async function renderQR() {
   await QRCode.toCanvas(qrCanvas.value, qrUrl.value, {
     width: 220,
     margin: 2,
-    errorCorrectionLevel: logoSrc ? 'H' : 'M',
+    errorCorrectionLevel: logoSrc ? 'M' : 'L',
   })
   if (!logoSrc) return
   const canvas = qrCanvas.value

frontend/src/components/payment/PaymentStatusPanel.vue

@@ -199,7 +199,7 @@ async function renderQR() {
   if (!qrCanvas.value || !qrUrl.value) return
   await QRCode.toCanvas(qrCanvas.value, qrUrl.value, {
     width: 220, margin: 2,
-    errorCorrectionLevel: 'H',
+    errorCorrectionLevel: 'M',
   })
 }
 

frontend/src/views/user/PaymentQRCodeView.vue

@@ -94,12 +94,12 @@ async function renderQR() {
   await nextTick()
   if (!qrCanvas.value || !qrUrl.value) return
 
-  // Use high error correction to support logo overlay
+  // Use medium error correction to support logo overlay while keeping QR code scannable
   const logoSrc = getLogoForType()
   await QRCode.toCanvas(qrCanvas.value, qrUrl.value, {
     width: 256,
     margin: 2,
-    errorCorrectionLevel: logoSrc ? 'H' : 'M',
+    errorCorrectionLevel: logoSrc ? 'M' : 'L',
   })
 
   if (!logoSrc) return