chore(docs): implement documentation

docs/user/multi-user.md

@@ -0,0 +1,20 @@
+## Multi-user
+
+Authentication is optional. When enabled:
+
+- Login/Register
+  - Local accounts with username/password
+  - First registered user becomes admin
+  - Public registration can be disabled
+- SSO (optional)
+  - Google and GitHub when configured
+- Roles
+  - User: can search/download, manage their profile
+  - Admin: access to all Configuration tabs and user management
+- Admin actions
+  - Create/delete users, change roles
+  - Reset user passwords
+
+Where to find it in the UI:
+- User menu (top-right) → Profile settings
+- Configuration → User Management (admin)