do not start fullscreen in nonqubes

[sssoleileraaa]

Description

Qubick off-sprint PR to no longer start the client in fullscreen.

Test Plan

• Start the client in non-qubes and verify that the client shows up max-window instead of fullscreen
• Start the client in qubes and verify that the client shows up max-window and that Part of the client window is hidden when resized #1109 is not reintroduced

[eloquence]

Works as intended in my Ubuntu env, but when I attempt to run the client from this branch in sd-app via python -m securedrop_client, the window never opens after the login screen (tried in offline mode). No error in logs/stdout/stderr, and running from an earlier commit works.

[sssoleileraaa]

whoops, left out one critical line, fixed!

[eloquence]

LGTM in both environments now, though looks like we have two legit test failures:

tests/gui/test_main.py::test_show_main_window_without_username FAILED [ 15%]
tests/gui/test_main.py::test_show_main_window FAILED [ 70%]

and a drop in coverage

securedrop_client/gui/main.py 94 5 95% 105-111

[sssoleileraaa]

thanks for the review! yes, showing the main windows needs coverage for both qubes and non-qubes now. this has been updated so we should see ci go green

[emkll]

Rebased on latest main

[emkll]

Thanks @creviera , LGTM based on test plan:

• Start the client in non-qubes and verify that the client shows up max-window instead of fullscreen
• Start the client in qubes and verify that the client shows up max-window and that Part of the client window is hidden when resized #1109 is not reintroduced
• Test coverage is 100%