In flat Vulkan mode, Steam Frame controller buttons can appear not to
reach the game while the system's laser-pointer mode still owns focus.
This is an easy mistake to make when starting a game from the headset.
After a successful boot, show an informational dialog for explicit Flat
Display (Vulkan) mode and for legacy configurations with OpenXR disabled.
The check happens after boot metadata and per-game VR settings are loaded,
so immersive VR and Stereo 3D Screen launches are not interrupted.
Add the requested reminder text and a Never See This Dialog Again
checkbox. Persist the opt-out in Main.Interface, and leave it unchecked by
default so the reminder appears for new users.
