linux: improve Wayland dialog surfacing and layout stability

[dragonleopardpig]

Summary

• improve surfacing/raising of QML dialogs on Wayland and tiling WMs
• fix add-sync / add-backup dialog visibility problems
• remove QML layout feedback loops that made these dialogs unstable

Problem
Several QML dialogs were created successfully but could still end up hidden,
collapsed, or otherwise unusable under Hyprland/Wayland. The most visible cases
were:

• Add sync
• Add backup
• onboarding-related dialog flows

There were two separate issues:

1. dialog surfacing and parenting paths were too fragile for this environment
2. some QML pages had layout bindings that caused recursive rearrange / polish
   loop failures

Fix

• improve dialog opener and wrapper behavior for these QML dialogs
• add explicit raise/surfacing paths where needed
• adjust sync/backups dialog handling and modality
• remove conflicting width/height/layout bindings from affected QML pages

Tested on

• NixOS 26.05
• Hyprland
• Qt 5.15