feat: Add database explorer page with SQLite management

- Add /database page with table explorer, query executor and stats
- Implement MCP tools: list_tables, get_table_schema, get_table_data,
  get_database_stats, execute_query
- Add database API endpoints with security (SELECT only)
- Add database icon to toolbar

frontend/src/router/index.ts

@@ -33,6 +33,11 @@ const router = createRouter({
       path: '/themes',
       name: 'themes',
       component: () => import('../pages/ThemesPage.vue')
+    },
+    {
+      path: '/database',
+      name: 'database',
+      component: () => import('../pages/DatabasePage.vue')
     }
   ]
 })