What’s the top-left Button in KDE Windows

Go to System Settings → Window Behaviour → Window Rules. Click on the ‘New…’ button. Add a description like ‘maximise everything on start’. Go to the ‘Size & Position’ tab, then enable the checkboxes for ‘Maximised horizontally’ and ‘Maximised vertically’. For each of them, set the drop-down to ‘Apply Initially’, and the radio button to ‘Yes’. Click ‘OK’, then ‘Apply’.

Now all windows will open maximised.

This includes windows you may not have thought of like docks, transient windows, dialogue boxes etc. You can exclude any of them by editing the rule you just made: go to the ‘Window matching’ tab for the rule, and unselect any window types you don't want affected. Try leaving just ‘Normal Window’.

The rule mechanism is very versatile. You can also modify the settings on a window-by-window or application-by-application basis, just click on a window titlebar's top-left icon, then go to Advanced → Special Window Settings or Special Application Settings. ‘Window’ makes a rule (or edits the existing one) for just that window, ‘Application’ for every window created by that application.

Update: you can also set the fullscreen status and/or remove window decorations with this mechanism. The window manager is your oyster.

Open KWalletManager (/usr/bin/kwalletmanager) in KDE, go to Contents / Form Data / Maps, select the correct id, check Show Values box. Works for me.

If you can't find it this way: save your login page to a HTML file, place it on a localhost webserver with PHP/CGI support, trick your resolver via /etc/hosts to call local page instead of the original, write a simple PHP/CGI script to display form data and attach it in saved HTML page under <form action=...>. Then login.