fix(ux): land quick-win UX audit fixes across settings, exports, toasts, nav

Batch of safe, isolated fixes from the 10-surface UX audit:

- settings: Auto-shield / Save-z-transactions toggles now persist on change
  (they discarded CB()'s return and relied on a Save button that only renders
  on the About tab, so the choice was silently lost on relaunch).
- exports: show the resolved Platform::getConfigDir() instead of a hardcoded
  ~/.config path (wrong on Windows/macOS) in both the all-keys and CSV
  dialogs; all-keys also reveals the file path on a partial export.
- notifications: error/warning toasts last 9s/7s (were 4s/3.5s) so failures
  are readable.
- qr popup / tx-details: copy-to-clipboard now confirms with a toast.
- about: Escape-to-close + readable heading contrast via OnSurfaceMedium().
- sidebar: suppress single-key nav (J/K/arrows/[) while an overlay dialog or
  ImGui popup is open, so it no longer switches the tab behind a modal.
- i18n: strip the stale hardcoded path out of file_save_location in i18n.cpp
  and all eight res/lang/*.json locales (additive, one key each).

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

src/ui/windows/about_dialog.cpp

@@ -38,6 +38,12 @@ void RenderAboutDialog(App* app, bool* p_open)
         return;
     }
 
+    // Escape closes the dialog (click-outside and the Close button already work; this matches the FAQ
+    // dialog's affordance so Escape behaves consistently across modal dialogs).
+    if (ImGui::IsKeyPressed(ImGuiKey_Escape)) {
+        *p_open = false;
+    }
+
     // Use Body2 font for all dialog text
     ImGui::PushFont(Type().body2());
 
@@ -107,7 +113,7 @@ void RenderAboutDialog(App* app, bool* p_open)
     ImGui::Spacing();
     
     // Credits
-    ImGui::TextColored(ImVec4(0.7f, 0.7f, 0.7f, 1.0f), "%s", TR("about_credits"));
+    ImGui::TextColored(ImGui::ColorConvertU32ToFloat4(material::OnSurfaceMedium()), "%s", TR("about_credits"));
     ImGui::Spacing();
     
     ImGui::BulletText("The Hush Developers");
@@ -121,7 +127,7 @@ void RenderAboutDialog(App* app, bool* p_open)
     ImGui::Spacing();
     
     // License
-    ImGui::TextColored(ImVec4(0.7f, 0.7f, 0.7f, 1.0f), "%s", TR("about_license"));
+    ImGui::TextColored(ImGui::ColorConvertU32ToFloat4(material::OnSurfaceMedium()), "%s", TR("about_license"));
     ImGui::Spacing();
     ImGui::TextWrapped("%s", TR("about_license_text"));