linux: improve Wayland tray popup behavior

[dragonleopardpig]

Summary

• treat the tray status popup as a regular window on native Wayland
• avoid popup-dismiss behavior that depends on unstable tray-host focus transitions
• keep the tray icon visible with a Linux fallback on Hyprland-like sessions

Problem
On native Wayland/Hyprland, left-clicking the tray icon opens the MEGAsync status
popup, but moving the pointer into the popup immediately dismisses it. The app
uses an emulated popup path on Linux that hides the window on focus changes.
That works on X11/XWayland, but native Wayland tray-host focus transitions are
not stable enough for this logic.

Fix

• detect native Wayland at runtime from Qt's platform name
• use the regular-window path for the status popup on native Wayland
• keep the existing emulated-popup path for X11/XWayland
• keep a tray icon fallback for Hyprland-like environments where themed tray
  assets are not resolved reliably

Tested on

• NixOS 26.05
• Hyprland
• Qt 5.15