mobile: Use titles style and single back button
This was removed in https://invent.kde.org/plasma-mobile/angelfish/-/commit/212ad75260d2ce0646175abfeb05e525b3e856f0.
I think that bringing it back makes sense because the title becomes a lot less cluttered, gives user an indication as to how to return to the content, while allowing us to keep the swipe gesture.
Before:
After:
Edited by Devin Lin