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/export_all_keys_dialog.cpp

@@ -93,7 +93,10 @@ void ExportAllKeysDialog::render(App* app)
         ImGui::Spacing();
         m::LabeledInput(TR("output_filename"), "##Filename", s_filename, sizeof(s_filename));
         ImGui::Spacing();
-        ImGui::TextColored(ImGui::ColorConvertU32ToFloat4(m::OnSurfaceDisabled()), "%s", TR("file_save_location"));
+        // Show the real, resolved config directory (not a hardcoded ~/.config path, which was wrong on
+        // Windows/macOS) so the user knows where this plaintext key file lands before they export it.
+        ImGui::TextColored(ImGui::ColorConvertU32ToFloat4(m::OnSurfaceDisabled()), "%s %s",
+                           TR("file_save_location"), util::Platform::getConfigDir().c_str());
         ImGui::EndDisabled();
 
         // Progress / result.
@@ -232,7 +235,10 @@ void ExportAllKeysDialog::render(App* app)
                                 Notifications::instance().error(TR("export_keys_write_fail"));
                             } else if (exported < total) {
                                 snprintf(buf, sizeof(buf), TR("export_keys_partial"), exported, total);
-                                s_status = buf;
+                                // A partial export still wrote a plaintext key file to disk — show WHERE,
+                                // so the user can find it to move or delete it (previously only a full
+                                // success revealed the path, leaving partial-export files orphaned).
+                                s_status = std::string(buf) + "\n" + filepath;
                                 snprintf(buf, sizeof(buf), TR("export_keys_partial_toast"), exported, total);
                                 Notifications::instance().warning(buf);
                             } else {