fix(settings): securely wipe the exported private key (no heap-lingering secret)

The Export Private Key dialog held the exported spending key in a plain
std::string cleared only with .clear() — never sodium_memzero'd — and on
outside-click/Esc the overlay's early return skipped even that, so the secret
lingered in freed heap. Hold it in a fixed char[256] and sodium_memzero on every
close path (Close button, outside-click/Esc early return, address change). Guard
the async export callback so it can't populate the buffer after the dialog closed.

Standalone security fix ahead of the settings-modal redesign (Phase 0a); the
dialog's full restyle is Phase 2.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.h

@@ -808,7 +808,7 @@ private:
     size_t daemon_output_offset_ = 0;      // for incremental output parsing (rescan detection)
     
     // Export/Import state
-    std::string export_result_;
+    char export_result_[256] = {0};   // SECRET exported key — fixed buffer so it can be sodium_memzero'd
     char import_key_input_[512] = {0};
     std::string export_address_;
     std::string import_status_;