Re-sync README 'System Dependencies' to the corrected INSTALL.md: drop the nonexistent gstreamer1-plugin-pipewire (Fedora) and gir1.2-gdk-4.0 (Debian), add the cairo build toolchain + xdg-desktop-portal + pipx, and add an openSUSE block (mirrors INSTALL.md's verified recipe). Add gstreamer1.0-tools to the Debian apt line in both README and INSTALL.md — it provides gst-launch-1.0, which screencast.py shells out to for screen recording and is not pulled in transitively on debian:trixie. PR #2 by @ha-ye.