feat: add database, role & privilege management

Add Admin sidebar tab with database/role management panels, role manager
workspace tab, and privilege dialogs. Backend provides 10 new Tauri
commands for CRUD on databases, roles, and privileges with read-only
mode enforcement. Context menus on schema tree nodes allow dropping
databases and viewing/granting table privileges.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/models/mod.rs

@@ -1,4 +1,5 @@
 pub mod connection;
 pub mod history;
+pub mod management;
 pub mod query_result;
 pub mod schema;