refactor(ui): add material::LabeledInput helper + adopt at 8 form fields

UI-standardization audit: the labeled single-line form field (a plain
ImGui::Text label above a fixed-width InputText/InputTextWithHint) was
open-coded across dialogs. Add material::LabeledInput(label, id, buf, size,
width=-1, hint=nullptr, flags=0) mirroring that idiom exactly — pixel-identical,
but a single chokepoint so input styling (label typography, frame, spacing) can
later evolve in one place. Returns the input's edited bool.

Adopt at the clean vertical (label-above) single-line sites: address-book
add/edit (label + address), shield from-address, export-all-keys + export-
transactions filename, validate-address input, request-payment label + URI.
Left as-is: horizontal label-beside-input rows (settings RPC host/port/user/
pass use Text + SameLine), multiline (InputTextMultiline), numeric (InputDouble),
combos, and hint-only inputs — none match the vertical single-line idiom.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/windows/request_payment_dialog.cpp

@@ -197,9 +197,7 @@ void RequestPaymentDialog::render(App* app)
         ImGui::Spacing();
 
         // Label (optional)
-        ImGui::Text("%s", TR("request_label"));
-        ImGui::SetNextItemWidth(-1);
-        if (ImGui::InputText("##Label", s_label, sizeof(s_label))) {
+        if (material::LabeledInput(TR("request_label"), "##Label", s_label, sizeof(s_label))) {
             s_uri_dirty = true;
         }
         
@@ -242,13 +240,11 @@ void RequestPaymentDialog::render(App* app)
 
         // Payment URI display
         if (!s_payment_uri.empty()) {
-            ImGui::Text("%s", TR("request_payment_uri"));
-            ImGui::SetNextItemWidth(-1);
-            
             // Use a selectable text area for the URI
             char uri_buf[1024];
             strncpy(uri_buf, s_payment_uri.c_str(), sizeof(uri_buf) - 1);
-            ImGui::InputText("##URI", uri_buf, sizeof(uri_buf), ImGuiInputTextFlags_ReadOnly);
+            material::LabeledInput(TR("request_payment_uri"), "##URI", uri_buf, sizeof(uri_buf),
+                                   -1.0f, nullptr, ImGuiInputTextFlags_ReadOnly);
             
             ImGui::Spacing();