When SSHing into a machine with existing tmux sessions, show an fzf
picker instead of always creating a new session. The picker shows
session name, window count, creation time, and attach status.

- Preview pane shows the current content of each session (via capture-pane)
- DEL key kills the highlighted session and refreshes the list
- Selecting a session attaches to it
- Selecting '>>> new session <<<' or pressing ESC creates a new one
- Local terminals are unaffected, only triggers when SSH_CLIENT is set

Add SABnzbd as Usenet download client alongside Transmission for torrents.
Radarr and Sonarr can now use both download methods.

- Add transmission-gc timer to remove completed torrents after ratio >= 5.0
or age >= 30 days, deleting files to free disk space
- Enable ratio-limit and seed-queue in transmission settings
- Fix transmission-watcher to check service state before restarting,
preventing restart loops during startup

The session keyring (@s) wasn't persisting keys when accessed from
systemd user services. Switch to user keyring (@u) which is shared
across all processes for the same UID.