Add callsign uniqueness checks and prevent demotion of the last admin. Introduces ECALLSIGNEXISTS and ELASTADMIN constants, new model methods (exists_by_callsign, count_admin_users, would_remove_last_admin) and integrates checks into add/update flows to return appropriate error codes. Update User controller to add validation callbacks (check_unique_callsign, check_last_admin_role) and wire errors into multiple user actions. Update signup/edit views to display callsign and usertype validation feedback using Bootstrap invalid-feedback and improve alert layout.
Introduces open registration settings in the options area, allowing administrators to enable or disable public user signup. Adds a registration options page, updates sidebar navigation, and implements a public signup form and controller logic. Also refines user model to ensure correct data types for user fields and updates language strings for registration. The login page now links to signup when open registration is enabled.
