CategoryView: absorb headerbar

[danirabbit]

Follow up to #278 as part of my evil plan to flatten settings. Plugins will now be responsible for providing window controls and a back button

Window controls in each plugin:

• [x] https://github.com/elementary/switchboard-plug-useraccounts/pull/223
• [x] https://github.com/elementary/switchboard-plug-printers/pull/202
• [x] https://github.com/elementary/switchboard-plug-network/pull/396
• [x] https://github.com/elementary/switchboard-plug-locale/pull/188
• [x] https://github.com/elementary/switchboard-plug-notifications/pull/100
• [x] https://github.com/elementary/switchboard-plug-parental-controls/pull/191
• [x] https://github.com/elementary/switchboard-plug-applications/pull/214
• [x] https://github.com/elementary/switchboard-plug-pantheon-shell/pull/389
• [x] https://github.com/elementary/switchboard-plug-security-privacy/pull/173
• [x] https://github.com/elementary/switchboard-plug-display/pull/380
• [x] https://github.com/elementary/switchboard-plug-keyboard/pull/503
• [x] https://github.com/elementary/switchboard-plug-mouse-touchpad/pull/232
• [x] https://github.com/elementary/switchboard-plug-power/pull/263
• [x] https://github.com/elementary/switchboard-plug-sound/pull/268
• [x] https://github.com/elementary/switchboard-plug-bluetooth/pull/218
• [x] https://github.com/elementary/switchboard-plug-wacom/pull/63
• [x] https://github.com/elementary/switchboard-plug-onlineaccounts/pull/279
• [x] https://github.com/elementary/switchboard-plug-sharing/pull/71
• [x] https://github.com/elementary/switchboard-plug-datetime/pull/122
• [x] https://github.com/elementary/switchboard-plug-about/pull/314

[danirabbit]

@leolost2605 can you post a screenshot? It looks the same here

[leolost2605]

Compared to

[danirabbit]

@leolost2605 oh wow haha. Can you make sure you're on io.elementary.stylesheet 7.3.0+r524+pkg338~daily~ubuntu24.04.1?

Also you might need to run ninja clean to make sure to rebuild CSS in this branch