feat: initial noctalia-greeter release

QuickShell QML login greeter for greetd, styled after noctalia-shell
lockscreen. Runs under niri compositor as _greeter user. Theme is
live-synced from noctalia config via runit service.

- shell.qml: QuickShell greeter UI + greetd auth flow
- sync/: inotifywait theme sync daemon + runit service
- greetd/: niri compositor config + wrapper scripts
- install.sh: deployment helper
- test/check-syntax.sh: 19 syntax/structural checks (0 failures)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

greetd/config.toml.example

@@ -0,0 +1,6 @@
+[terminal]
+vt = 7
+
+[default_session]
+command = "/usr/sbin/dbus-run-session /usr/sbin/niri --config /etc/greetd/niri-greeter.kdl"
+user = "_greeter"