Add GNOME module, centralize Wayland portals, set default Ly session, and remove camera portal

[moons-14]

Motivation

• Provide an optional GNOME desktop module and enable it as part of the desktop feature set.
• Centralize Wayland xdg-portal backend configuration so hardware/app modules don't append their own portals.
• Ensure a sensible default session for the ly display manager and remove redundant portal configuration from the camera module.

Description

• Added a new modules/applications/gnome.nix module that exposes my.applications.gnome.enable, enables services.desktopManager.gnome, disables GDM, and installs gnome-tweaks and gnome-extension-manager into environment.systemPackages when enabled.
• Imported the new gnome.nix in modules/applications/default.nix and enabled my.applications.gnome by default from modules/features/gui/desktop.nix.
• Updated modules/applications/ly/default.nix to set services.displayManager.defaultSession to niri when ly is enabled.
• Reworked modules/applications/wayland.nix to centralize xdg.portal config, adding xdg-desktop-portal-gnome to extraPortals, reordering portals, and defining backend defaults (common.default, niri.default, and gnome.default).
• Removed the xdg.portal block from modules/system/camera.nix to avoid duplicating portal configuration.

Testing

• No automated tests were run for this change.

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Codex Task