feat: implement user authentication and permission-based data filtering

Add user login system with role-based access control:
- Admin users see all data and can filter by any manager
- Regular users only see records where ManagerName matches their username

New components:
- BIPUsersDAO: user authentication and management
- SessionManager: singleton session state management
- LoginDialog: modal login UI for app startup
- init_users.sql: initial user setup script

Permission enforcement:
- Material validation tab: hide manager filter for non-admin, force filter by current user
- Material type management: hide filter UI for non-admin, filter at database level
- Window title and status bar display current user info

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

auth/__init__.py

@@ -0,0 +1,6 @@
+"""
+Auth package for user authentication and session management
+"""
+from .session_manager import SessionManager
+
+__all__ = ['SessionManager']

auth/session_manager.py

@@ -0,0 +1,130 @@
+"""
+Session Manager - Singleton pattern for managing authenticated user session
+"""
+from typing import Optional
+
+
+class SessionManager:
+    """
+    Singleton session manager to maintain authenticated user state
+    throughout the application lifecycle.
+    """
+
+    _instance = None
+
+    def __new__(cls):
+        """Implement singleton pattern"""
+        if cls._instance is None:
+            cls._instance = super(SessionManager, cls).__new__(cls)
+            cls._instance._initialized = False
+        return cls._instance
+
+    def __init__(self):
+        """Initialize the session manager (only once)"""
+        if self._initialized:
+            return
+
+        self._current_user = None  # {username, user_type}
+        self._initialized = True
+
+    @classmethod
+    def get_instance(cls) -> 'SessionManager':
+        """
+        Get the singleton instance of SessionManager
+
+        Returns:
+            The singleton SessionManager instance
+        """
+        if cls._instance is None:
+            cls._instance = cls()
+        return cls._instance
+
+    def login(self, username: str, password: str) -> bool:
+        """
+        Authenticate and login a user
+
+        Args:
+            username: The username to authenticate
+            password: The password to verify
+
+        Returns:
+            True if login successful, False otherwise
+        """
+        from db.bip_users_dao import BIPUsersDAO
+
+        dao = BIPUsersDAO()
+        user_info = dao.authenticate(username, password)
+
+        if user_info:
+            self._current_user = {
+                'username': user_info['username'],
+                'user_type': user_info['user_type']
+            }
+            return True
+        return False
+
+    def logout(self):
+        """Logout the current user and clear session"""
+        self._current_user = None
+
+    def is_authenticated(self) -> bool:
+        """
+        Check if a user is currently authenticated
+
+        Returns:
+            True if user is logged in, False otherwise
+        """
+        return self._current_user is not None
+
+    def is_admin(self) -> bool:
+        """
+        Check if the current user is an admin
+
+        Returns:
+            True if current user is admin, False otherwise
+        """
+        if not self._current_user:
+            return False
+        return self._current_user.get('user_type') == 'Admin'
+
+    def is_guest(self) -> bool:
+        """
+        Check if the current user is a guest
+
+        Returns:
+            True if current user is guest, False otherwise
+        """
+        if not self._current_user:
+            return False
+        return self._current_user.get('user_type') == 'Guest'
+
+    def get_username(self) -> Optional[str]:
+        """
+        Get the current username
+
+        Returns:
+            Current username if authenticated, None otherwise
+        """
+        if not self._current_user:
+            return None
+        return self._current_user.get('username')
+
+    def get_user_type(self) -> Optional[str]:
+        """
+        Get the current user type
+
+        Returns:
+            Current user type if authenticated, None otherwise
+        """
+        if not self._current_user:
+            return None
+        return self._current_user.get('user_type')
+
+    def get_user_info(self) -> Optional[dict]:
+        """
+        Get all current user information
+
+        Returns:
+            Dict with username and user_type if authenticated, None otherwise
+        """
+        return self._current_user