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/layout/Sidebar.tsx

@@ -2,9 +2,10 @@ import { useState } from "react";
 import { Input } from "@/components/ui/input";
 import { SchemaTree } from "@/components/schema/SchemaTree";
 import { HistoryPanel } from "@/components/history/HistoryPanel";
+import { AdminPanel } from "@/components/management/AdminPanel";
 import { Search } from "lucide-react";
 
-type SidebarView = "schema" | "history";
+type SidebarView = "schema" | "history" | "admin";
 
 export function Sidebar() {
   const [view, setView] = useState<SidebarView>("schema");
@@ -33,6 +34,16 @@ export function Sidebar() {
         >
           History
         </button>
+        <button
+          className={`flex-1 px-3 py-1.5 font-medium ${
+            view === "admin"
+              ? "bg-background text-foreground"
+              : "text-muted-foreground hover:text-foreground"
+          }`}
+          onClick={() => setView("admin")}
+        >
+          Admin
+        </button>
       </div>
 
       {view === "schema" ? (
@@ -52,8 +63,10 @@ export function Sidebar() {
             <SchemaTree />
           </div>
         </>
-      ) : (
+      ) : view === "history" ? (
         <HistoryPanel />
+      ) : (
+        <AdminPanel />
       )}
     </div>
   );