v0.7.0 — Debian/apt support

Plaza now supports Debian/apt as a full source, alongside Arch (pacman + AUR) and Flatpak.

• apt search (apt-cache search + policy), detail (apt-cache show), and install/upgrade/remove (apt-get)
• apt in the Manage view: installed list with origin/size/install date, explicit and orphan filters, sort, remove, per-source and per-package upgrade
• Arch-specific controls are hidden when not on Arch (and vice versa), so Plaza matches the system it runs on

204 tests, clippy clean.