Config file, user service, and a build container that can see bluez

`--config` reads the same directives as the command line, one per line with
the rest of the line as the command, so a unit can carry the kiosk's events
without shell quoting. presence.service is a systemd --user unit that runs it
from ~/.config/presence/config, restarts on failure, and only starts when the
file exists; config.example is the starting point.

The x86_64 release job failed on makepkg's runtime dependency check because
bluez was not in the container. It is now.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

PKGBUILD

@@ -28,4 +28,6 @@ package() {
     install -Dm755 target/release/presence "$pkgdir/usr/bin/presence"
     install -Dm644 README.md "$pkgdir/usr/share/doc/presence/README.md"
     install -Dm644 LICENSE "$pkgdir/usr/share/licenses/presence/LICENSE"
+    install -Dm644 config.example "$pkgdir/usr/share/doc/presence/config.example"
+    install -Dm644 presence.service "$pkgdir/usr/lib/systemd/user/presence.service"
 }