fix: audit MEDIUM fixes — FP race, async avatar, symlink, FD leak (v0.8.5)

- i18n test: assert four previously-missing string fields so future locales
  cannot ship empty strings unnoticed.
- greeter: atomic check-and-set fingerprint_probe_initializing to keep a
  fast user switch from spawning two parallel fprintd D-Bus inits.
- greeter: set_avatar_from_file decodes via gio::File::read_future +
  Pixbuf::from_stream_at_scale_future inside glib::spawn_future_local;
  shows default icon first, swaps on completion.
- greeter: cap MAX_WALLPAPER_FILE_SIZE at 10 MB and MAX_AVATAR_FILE_SIZE
  at 5 MB to bound worst-case decode latency.
- config: apply the same symlink-rejection check to the Moonarch
  wallpaper fallback that the user-configured path already uses.
- greeter: after login_worker returns, drop the cloned greetd socket
  held in shared state so repeated failed logins do not leak FDs.

src/i18n.rs

@@ -286,6 +286,10 @@ mod tests {
             assert!(!s.faillock_attempts_remaining.is_empty(), "{locale}: faillock_attempts_remaining");
             assert!(!s.faillock_locked.is_empty(), "{locale}: faillock_locked");
             assert!(!s.unexpected_greetd_response.is_empty(), "{locale}: unexpected_greetd_response");
+            assert!(!s.greetd_sock_not_absolute.is_empty(), "{locale}: greetd_sock_not_absolute");
+            assert!(!s.invalid_session_command.is_empty(), "{locale}: invalid_session_command");
+            assert!(!s.session_start_failed.is_empty(), "{locale}: session_start_failed");
+            assert!(!s.socket_error.is_empty(), "{locale}: socket_error");
         }
     }