…been shown

009104c fix: return full functionality of Back / Cancel button on mnemonic verification dialog (Konstantin Akimov)
c24473b fix: crash in mnemonicverificationdialog by proper using reject() event (Konstantin Akimov)

Pull request description:

  ## Issue being fixed or feature implemented
  First found by thepez while testing #7040

  It happens every time when create new wallet after dialog to validate mnemonic has been shown.

  Steps to reproduce:
   1. Create new wallet
   2. Show mnemonic
   3. Confirm the mnemonic is saved
   4. Close dialog by Canceling validation or by Confirming validation
   5. Change theme crashes app

  ```
      2025-12-22T15:16:31Z Posix Signal: Segmentation fault
      0#: (0x608F5BE3FDB5) stl_vector.h:115         - std::_Vector_base<unsigned long, std::allocator<unsigned long> >::_Vector_impl_data::_M_copy_data(std::_Vector_base<unsigned long, std::allocator<unsigned long> >::_Vector_impl_data const&)
       1#: (0x608F5BE3FDB5) stl_vector.h:127         - std::_Vector_base<unsigned long, std::allocator<unsigned long> >::_Vector_impl_data::_M_swap_data(std::_Vector_base<unsigned long, std::allocator<unsigned long> >::_Vector_impl_data&)
       2#: (0x608F5BE3FDB5) stl_vector.h:1962        - std::vector<unsigned long, std::allocator<unsigned long> >::_M_move_assign(std::vector<unsigned long, std::allocator<unsigned long> >&&, std::integral_constant<bool, true>)
       3#: (0x608F5BE3FDB5) stl_vector.h:771         - std::vector<unsigned long, std::allocator<unsigned long> >::operator=(std::vector<unsigned long, std::allocator<unsigned long> >&&)
       4#: (0x608F5BE3FDB5) stacktraces.cpp:784      - HandlePosixSignal
       5#: (0x7D6B37A45330) libc_sigaction.c         - ???
       6#: (0x608F5CC0951B) <unknown-file>           - ???
       7#: (0x608F5CC09901) <unknown-file>           - ???
       8#: (0x608F5B62369D) unique_lock.h:105        - std::unique_lock<std::recursive_mutex>::~unique_lock()
       9#: (0x608F5B62369D) sync.h:226               - UniqueLock<AnnotatedMixin<std::recursive_mutex> >::~UniqueLock()
      10#: (0x608F5B62369D) guiutil.cpp:1031         - GUIUtil::loadStyleSheet(bool)
      11#: (0x608F5B6243C4) guiutil.cpp:1616         - GUIUtil::loadTheme(bool)
      12#: (0x608F5B6C27B7) atomic_base.h:505        - std::__atomic_base<int>::load(std::memory_order) const
      13#: (0x608F5B6C27B7) qatomic_cxx11.h:239      - int QAtomicOps<int>::loadRelaxed<int>(std::atomic<int> const&)
      14#: (0x608F5B6C27B7) qbasicatomic.h:107       - QBasicAtomicInteger<int>::loadRelaxed() const
      15#: (0x608F5B6C27B7) qrefcount.h:66           - QtPrivate::RefCount::deref()
      16#: (0x608F5B6C27B7) qstring.h:1308           - QString::~QString()
      17#: (0x608F5B6C27B7) appearancewidget.cpp:110 - AppearanceWidget::updateTheme(QString const&)
      18#: (0x608F5C90A0CD) <unknown-file>           - ???
      19#: (0x608F5CC5B1E1) <unknown-file>           - ???
      20#: (0x608F5CC5D00B) <unknown-file>           - ???
      21#: (0x608F5CC5D29B) <unknown-file>           - ???
      22#: (0x608F5C90A2EB) <unknown-file>           - ???
      23#: (0x608F5CC57376) <unknown-file>           - ???
      24#: (0x608F5CC586F6) <unknown-file>           - ???
      25#: (0x608F5C8DD693) <unknown-file>           - ???
  ```

  Current call `disconnect(cancel, nullptr, nullptr, nullptr)` is too violent for qt objects because it disconnects not only user-added slots, but internal qt slots too; it makes the object _Cancel Button_ to be in an invalid state.

  ## What was done?
  Easiest fix is specify _some_ arguments for disconnect, for example, simple `disconnect(cancel, &QDialogButtonBox::rejected, this, nullptr);` fixes crash but it breaks functionality of Cancel button.

  For the final solution look to the commits in PR

  ## How Has This Been Tested?

  Tested numerous user cases (list is not full):
   - create wallet, validate mnemonic, click "BACK"
   - create wallet, close dialog without validation mnemonic
   - create wallet, validate mnemonic, click BACK, re-validate mnemonic
   - show mnemonic for already existing wallet

  Also I tried to do the exactly same things, but instead mouse's click use Escape or Ctrl+W or close window by mouse.

  I am not sure that I provided 100% coverage by manual testing, but at this point it looks solid for me. Any extra testing is welcome, please help with it!

  ## Breaking Changes
  N/A

  ## Checklist:
  - [x] I have performed a self-review of my own code
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have added or updated relevant unit/integration/functional/e2e tests
  - [ ] I have made corresponding changes to the documentation
  - [x] I have assigned this pull request to a milestone _(for repository code-owners and collaborators only)

ACKs for top commit:
  UdjinM6:
    ACK 009104c

Tree-SHA512: aab64388dcc90b0bd2ab035fc11f0b53c55643dcf3ed6c0859ba2ce3865d8341243892e56fc998391652b9da11be815360c80314c24c6cea90a896196a29f9ae