Add /qr route and refactor payment QR code generation

Introduce a dedicated /qr route for QR code preview via query params.
Add PaymentRequest model for payment data and validation, replacing
the old inline model. Extract QR code generation to PaymentQrCodeService
for reuse. Refactor Home.razor to use new model/service and improve
validation/UI. Add EmptyLayout for minimal layout needs. Update
.gitignore and _Imports.razor for new structure.

QRPay/Pages/Qr.razor.css

@@ -0,0 +1,46 @@
+.qr-route-page {
+    min-height: 100vh;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 24px;
+    background: #ffffff;
+}
+
+.qr-route-image {
+    display: block;
+    width: min(92vw, 420px);
+    height: auto;
+    aspect-ratio: 1;
+}
+
+.qr-route-error {
+    max-width: 420px;
+    padding: 24px;
+    border: 1px solid #fecaca;
+    border-radius: 16px;
+    background: #fef2f2;
+    color: #991b1b;
+    text-align: left;
+    box-shadow: 0 12px 30px rgba(0, 0, 0, 0.08);
+}
+
+.qr-route-error p {
+    margin: 0 0 8px;
+}
+
+.qr-route-error p:last-child {
+    margin-bottom: 0;
+}
+
+.visually-hidden {
+    position: absolute;
+    width: 1px;
+    height: 1px;
+    padding: 0;
+    margin: -1px;
+    overflow: hidden;
+    clip: rect(0, 0, 0, 0);
+    white-space: nowrap;
+    border: 0;
+}