feat(auth): Add optional authentication system (#22)

Backend:
- Add User model with username, hashed password, role, is_active
- Add passlib[bcrypt] and python-jose[cryptography] dependencies
- Create auth.py with JWT utilities, password hashing, role checking
- Add /auth endpoints: status, setup, login, me, users CRUD
- Add role-based permission system (viewer/operator/editor/admin)
- Environment: SPOOLMAN_AUTH_ENABLED, SPOOLMAN_AUTH_SECRET_KEY
- Migration for user table

Frontend:
- Add AuthProvider context with login/logout/setup
- Add LoginPage component with setup mode for first user
- Add ProtectedRoute wrapper for auth-required pages
- Add axios interceptor to attach JWT token to requests
- Add User Management tab in Settings (admin only)
- Add translation keys for auth UI

When SPOOLMAN_AUTH_ENABLED=true:
- Users must login to access the app
- First visit shows setup page to create admin account
- Role-based access control for future endpoint protection

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

client/public/locales/en/common.json

@@ -420,6 +420,39 @@
             "key_not_changed": "Please change the key to something else.",
             "delete_confirm": "Delete field {{name}}?",
             "delete_confirm_description": "This will delete the field and all associated data for all entities."
+        },
+        "users": {
+            "tab": "Users",
+            "title": "User Management",
+            "add": "Add User",
+            "edit": "Edit User",
+            "username": "Username",
+            "password": "Password",
+            "role": "Role",
+            "status": "Status",
+            "actions": "Actions",
+            "active": "Active",
+            "inactive": "Inactive",
+            "you": "You",
+            "created": "User created successfully",
+            "updated": "User updated successfully",
+            "deleted": "User deleted successfully",
+            "delete_confirm": "Are you sure you want to delete this user?",
+            "username_required": "Username is required",
+            "username_min": "Username must be at least 3 characters",
+            "password_required": "Password is required",
+            "password_min": "Password must be at least 6 characters",
+            "password_change_hint": "Leave blank to keep current password",
+            "leave_blank": "Leave blank to keep current",
+            "role_viewer": "Viewer - Can only view data",
+            "role_operator": "Operator - Can view and record usage",
+            "role_editor": "Editor - Can view, use, create, and edit",
+            "role_admin": "Admin - Full access including user management",
+            "auth_disabled": "Authentication is not enabled",
+            "auth_disabled_hint": "Set SPOOLMAN_AUTH_ENABLED=true to enable user management",
+            "admin_required": "Admin Access Required",
+            "admin_required_hint": "Only administrators can manage users",
+            "logout": "Log Out"
         }
     },
     "documentTitle": {