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/util/i18n.cpp

@@ -1606,7 +1606,7 @@ void I18n::loadBuiltinEnglish()
         strings_["date_label"] = "Date:";
         strings_["delete"] = "Delete";
         strings_["fee_label"] = "Fee:";
-        strings_["file_save_location"] = "File will be saved in: ~/.config/ObsidianDragon/";
+        strings_["file_save_location"] = "File will be saved in:";
         strings_["hide"] = "Hide";
         strings_["label"] = "Label:";
         strings_["loading"] = "Loading...";