Add authentication features for admin access

- Introduce a new `auth` module with login, logout, and user verification endpoints for a single admin user.
- Update backend routes to require admin authentication for sensitive operations, enhancing security.
- Modify frontend components to include an authentication provider and gate, ensuring only authorized users can access the application.
- Implement a login page for admin access, improving user experience and security management.
- Update API request handling to redirect unauthorized users to the login page, ensuring proper access control.

backend/routes/knowledge_bases.py

@@ -10,11 +10,16 @@ from db.models import KnowledgeBase, ModelResource
 from db.session import get_session
 from fastapi import APIRouter, Depends, HTTPException
 from schemas import KnowledgeBaseOut, KnowledgeBaseUpsert
+from services.auth import require_admin
 from sqlalchemy import select
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession
 
-router = APIRouter(prefix="/api/knowledge-bases", tags=["knowledge-bases"])
+router = APIRouter(
+    prefix="/api/knowledge-bases",
+    tags=["knowledge-bases"],
+    dependencies=[Depends(require_admin)],
+)
 
 
 async def _validate_embedding_resource(