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/components/workspace/TabContent.tsx

@@ -2,6 +2,7 @@ import { useAppStore } from "@/stores/app-store";
 import { WorkspacePanel } from "./WorkspacePanel";
 import { TableDataView } from "@/components/table-viewer/TableDataView";
 import { TableStructure } from "@/components/table-viewer/TableStructure";
+import { RoleManagerView } from "@/components/management/RoleManagerView";
 
 export function TabContent() {
   const { tabs, activeTabId, updateTab } = useAppStore();
@@ -43,6 +44,13 @@ export function TabContent() {
           table={activeTab.table!}
         />
       );
+    case "roles":
+      return (
+        <RoleManagerView
+          key={activeTab.id}
+          connectionId={activeTab.connectionId}
+        />
+      );
     default:
       return null;
   }