feat(configurator): migrate from Iced to GTK4 and libadwaita

[devmobasa]

Summary

• replace the Iced configurator frontend with GTK4 and libadwaita through Relm4
• port all settings pages while preserving search, deep links, validation, dirty-state tracking, and guarded configuration saves
• retain daemon setup, session management, migration, repair, and asynchronous filesystem behavior
• update CI, release packaging, desktop integration, and Nix dependencies for the new toolkit
• remove the configurator's obsolete Iced, Vulkan, and OpenGL dependencies

Compatibility

• keep the libadwaita API floor at 1.4, compatible with Ubuntu 24.04's libadwaita 1.5
• preserve --open destinations by allowing each launch to create its own application instance
• preserve comments, ordering, and unknown TOML settings through ConfigDocument
• keep revision-aware, atomic saves and existing conflict detection behavior