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/export_transactions_dialog.cpp

@@ -82,9 +82,7 @@ void ExportTransactionsDialog::render(App* app)
         ImGui::Spacing();
         
         // Filename
-        ImGui::Text("%s", TR("output_filename"));
-        ImGui::SetNextItemWidth(-1);
-        ImGui::InputText("##Filename", s_filename, sizeof(s_filename));
+        material::LabeledInput(TR("output_filename"), "##Filename", s_filename, sizeof(s_filename));
         
         ImGui::Spacing();
         ImGui::TextDisabled("%s", TR("file_save_location"));