Fix install docs: install-from-source, missing packages, GNOME note
Packages aren't on PyPI — document clone + pipx install from source.
Add Arch section with all required packages. Add gstreamer1-plugin-
pipewire to Fedora packages. Add note that activity detection (idle,
lock, power save) requires GNOME desktop — other desktops get capture
but no activity-based segment boundaries.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>