feat: add fprintd fingerprint authentication via greetd multi-stage PAM (v0.6.0)

Fingerprint auth was missing because moongreet rejected multi-stage
auth_message sequences from greetd. With pam_fprintd.so in the PAM
stack, greetd sends non-secret prompts for fingerprint and secret
prompts for password — moongreet now handles both in a loop.

- Replace single-pass auth with multi-stage auth_message loop
- fprintd D-Bus probe (gio::DBusProxy) for UI feedback only
- Fingerprint label shown when device available and fingers enrolled
- 60s socket timeout when fingerprint available (pam_fprintd scan time)
- Config option: [appearance] fingerprint-enabled (default: true)
- Fix: password entry focus loss after auth error (grab_focus while
  widget was insensitive — now re-enable before grab_focus)

README.md

@@ -15,6 +15,7 @@ Part of the Moonarch ecosystem.
 - **Multi-monitor** — Greeter on primary, wallpaper on all monitors
 - **i18n** — German and English (auto-detected from system locale)
 - **Faillock warning** — Warns after 2 failed attempts, locked message after 3
+- **Fingerprint** — fprintd support via greetd multi-stage PAM (configurable)
 
 ## Requirements