fix: Implement destroy method for StatusIconDummy

[City-busz]

This is needed to avoid error when the user disables the tray icon in the settings:

Traceback (most recent call last):
  File "/usr/lib/python3.14/site-packages/scc/gui/global_settings.py", line 387, in on_random_checkbox_toggled
    self.save_config()
    ~~~~~~~~~~~~~~~~^^
  File "/usr/lib/python3.14/site-packages/scc/gui/global_settings.py", line 309, in save_config
    self.app.destroy_statusicon()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib/python3.14/site-packages/scc/gui/app.py", line 294, in destroy_statusicon
    self.statusicon.destroy()
    ~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib/python3.14/site-packages/scc/gui/statusicon.py", line 380, in destroy
    self._status_fb.destroy()
    ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'StatusIconDummy' object has no attribute 'destroy'

[City-busz]

It's not needed if #112 is implemented.

[C0rn3j]

Not a bad idea to be a bit defensive either way, I pulled it, thanks.