fix: Audit-Findings — Theme-Validierung, locale-unabhängige Tests

- Theme-Name aus settings.ini gegen Regex validiert (nur [A-Za-z0-9_-]),
  verhindert Path-Traversal über GTK-Theme-Loading (S-05)
- Faillock-Tests nutzen expliziten strings-Parameter statt System-Locale,
  Tests laufen jetzt auch auf EN-Systemen (MAINT-4)
- Test für Path-Traversal im Theme-Namen ergänzt

src/moongreet/users.py

@@ -2,9 +2,12 @@
 # ABOUTME: Provides User dataclass and helper functions for the greeter UI.
 
 import configparser
+import re
 from dataclasses import dataclass
 from pathlib import Path
 
+VALID_THEME_NAME = re.compile(r"^[A-Za-z0-9_-]+$")
+
 NOLOGIN_SHELLS = {"/usr/sbin/nologin", "/sbin/nologin", "/bin/false", "/usr/bin/nologin"}
 MIN_UID = 1000
 MAX_UID = 65533
@@ -102,6 +105,9 @@ def get_user_gtk_theme(config_dir: Path | None = None) -> str | None:
         return None
 
     if config.has_option("Settings", "gtk-theme-name"):
-        return config.get("Settings", "gtk-theme-name")
+        theme = config.get("Settings", "gtk-theme-name")
+        # Validate against path traversal — only allow safe theme names
+        if theme and VALID_THEME_NAME.match(theme):
+            return theme
 
     return None