fix(request): clear the stale payment URI, show it only for an address

The request-payment dialog kept its generated payment URI across opens
and closes. A dialog opened for an address with no payable shielded
address could still show the previous request's URI, and the string
lingered after the dialog closed.

- The payment URI is cleared when the dialog opens, when it closes, and
  on the close cleanup path.
- The URI field is shown only when there is both a URI and a real
  address to pay.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/windows/request_payment_dialog.cpp

@@ -46,6 +46,7 @@ void RequestPaymentDialog::show(const std::string& address)
     s_amount = 0.0;
     s_memo[0] = '\0';
     s_label[0] = '\0';
+    s_payment_uri.clear();
     s_uri_dirty = true;
     
     // Only a shielded address can be paid (DragonX is a private chain): never prefill a mining address.
@@ -66,6 +67,7 @@ void RequestPaymentDialog::show(const std::string& address)
 void RequestPaymentDialog::hide()
 {
     s_open = false;
+    s_payment_uri.clear();
     if (s_qr_texture != 0) {
         FreeQRTexture(s_qr_texture);
         s_qr_texture = 0;
@@ -184,7 +186,7 @@ void RequestPaymentDialog::render(App* app)
         ImGui::Spacing();
 
         // Payment URI display
-        if (!s_payment_uri.empty()) {
+        if (!s_payment_uri.empty() && s_address[0] != '\0') {
             ImGui::Text("%s", TR("request_payment_uri"));
             // Bordered read-only field: bounds the raw drgx: URI within a box (it horizontal-scrolls if
             // long) rather than 4-char-chunking it like an address or letting a plain field overflow.
@@ -249,6 +251,9 @@ void RequestPaymentDialog::render(App* app)
     }
     
     // Cleanup on close
+    if (!s_open) {
+        s_payment_uri.clear();
+    }
     if (!s_open && s_qr_texture != 0) {
         FreeQRTexture(s_qr_texture);
         s_qr_texture = 0;