feat(keys): improve the key-export modal — auto-clear copy, inline QR, cleaner actions

- Auto-clear: the Copy button now routes through App::copySecretToClipboard, so a copied
  private/viewing key is wiped from the clipboard after ~45s (same protection as the seed) with
  a "auto-clears" notice — instead of the raw SetClipboardText that left it indefinitely.
- QR: once the key is revealed, a Show/Hide QR toggle renders the key's QR inline (via the same
  GenerateQRTexture/RenderQRCode widget the Receive tab uses) for scanning into another wallet.
  The QR texture is cached, regenerated on key change, and freed on hide/close/dismiss; hiding the
  key also hides its QR.
- Actions row tightened to Show/Hide · Copy · QR, and the key + QR texture are now cleared on any
  dismissal (Close button, scrim click, Esc), not just the Close button.

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

src/util/i18n.cpp

@@ -673,6 +673,8 @@ void I18n::loadBuiltinEnglish()
         strings_["address_details"] = "Address Details";
         strings_["view_on_explorer"] = "View on Explorer";
         strings_["qr_code"] = "QR Code";
+        strings_["show_qr"] = "Show QR";
+        strings_["hide_qr"] = "Hide QR";
         strings_["request_payment"] = "Request Payment";
         
         // Transactions Tab