feat: detect Flatpak terminals

[ericmacedo]

Summary

Apdatifier currently discovers terminal applications only with command -v. Flatpak terminals do not install host executables, so supported applications such as Flatpak Yakuake are absent from the terminal selector even though their session D-Bus interface is available.

This feature adds Flatpak-aware terminal support alongside native discovery:

• Scans installed Flatpak application desktop entries for supported terminal commands.
• Adds only Flatpak terminals that allow the org.freedesktop.Flatpak host-command bridge, preventing unusable sandboxed entries from appearing.
• Presents detected entries as command (Flatpak) and preserves their application ID and command in the selected value.
• Launches regular Flatpak terminals through flatpak run and executes Apdatifier maintenance commands on the host with flatpak-spawn --host.
• Starts Flatpak Yakuake when necessary, continues to use its existing D-Bus integration, and executes the maintenance command through the same host bridge.
• Forces TERM=xterm-256color for bridge commands so tput and clear work when Apdatifier is launched by Plasma without a usable terminal environment.
• Documents that supported terminals may be installed natively or through Flatpak.

Validation

• bash -n package/contents/tools/sh/terminal
• node --check package/contents/tools/tools.js
• Live discovery against the installed Flatpak Yakuake returned flatpak://org.kde.yakuake/yakuake.
• Verified flatpak-spawn --host is available in the Yakuake Flatpak.
• Verified the bridge can run tput cols and clear with TERM=xterm-256color.