feat: system knowledge base management with admin/user role separation

- Auto-create system KBs from data/knowledge_base/ subdirectories on startup
- Upgrade existing user KBs to system KBs when matching directory found
- Admin can upload to system KBs (saved to knowledge_base dir)
- Regular users see only their own KBs on knowledge page
- Fix API permission checks for system KBs (list, detail, documents, update, delete)
- Add is_superuser to UserResponse and frontend User type
- Fix sidebar session menu visibility for long titles

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/src/components/chat/export-dialog.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useState } from "react";
-import { Download, FileText, FileJson, File } from "lucide-react";
+import { Download, FileText, FileJson } from "lucide-react";
 import { Button } from "@/components/ui/button";
 import {
   Dialog,
@@ -61,12 +61,6 @@ export default function ExportDialog({ sessionId, sessionTitle, children, onClos
       description: "可读性好的文本格式，适合分享",
       icon: FileText,
     },
-    {
-      value: "pdf",
-      label: "PDF 格式",
-      description: "适合打印和正式文档",
-      icon: File,
-    },
   ];
 
   return (