fix(portal): screen-capture script, portal backend, swaync autostart

screen-capture.sh was a single non-executable line containing a `slur` typo,
so it never ran and nothing referenced it; rewritten as a working region
capture to ~/Pictures plus clipboard.

xdg-desktop-portal/portals.conf pins the wlr backend for sway, and swaync is
pulled in via graphical-session.target.wants so the portal's
Requisite=graphical-session.target is satisfied (the reason screen sharing
previously never started).

.config/systemd/user/graphical-session.target.wants/swaync.service

@@ -0,0 +1 @@
+/usr/lib/systemd/user/swaync.service