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/ui/windows/key_export_dialog.h

@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include <cstdint>
 #include <string>
 
 namespace dragonx {
@@ -47,10 +48,15 @@ private:
     static bool s_open;
     static bool s_fetching;
     static bool s_show_key;
+    static bool s_show_qr;            // inline QR of the revealed key (for scanning into another wallet)
+    static std::uintptr_t s_qr_tex;  // cached QR texture handle (0 = none)
+    static std::string s_qr_cached;  // key the cached QR was generated for
     static KeyType s_key_type;
     static std::string s_address;
     static std::string s_key;
     static std::string s_error;
+
+    static void releaseQr();  // free the QR texture + reset its state
 };
 
 } // namespace ui