services/polkit: register agent using XDG_SESSION_ID when available

[Kitty-Hivens]

qs_polkit_agent_register builds the agent subject with polkit_unix_session_new_for_process(getpid()), which resolves the logind session from the calling process's cgroup. When Quickshell runs outside a session-scope cgroup -- for example as a systemd user service -- there is no such session, subject creation fails, and the listener never registers ("failed to create subject for listener: No session for pid"). With no agent registered, polkit falls back to a text agent and authentication does not work for that session.

This prefers the session id from XDG_SESSION_ID and registers the listener for it, falling back to the existing pid-based path when the variable is unset. When the cgroup lookup fails, polkitd resolves the caller's session through sd_uid_get_display, so registration is accepted as long as XDG_SESSION_ID is the user's display session. This is the same approach hyprpolkitagent uses.

Tested on a session where the compositor and agent run as systemd user services: before, the agent failed to register and polkit fell back to the text agent; after, it registers and both GUI and pkexec authentication work.