refactor: reorganize admin settings components and remove deprecated files

[chenyme]

Summary

Refactor admin settings section file organization.

This change moves login authentication, conversation configuration, and files/RAG settings out of the generic sections/settings folder into dedicated admin section folders:

• sections/login
• sections/conversation
• sections/files

Shared admin section form components were moved to sections/shared. Route entry imports and related component imports were updated accordingly. This is a frontend structure-only refactor and does not change runtime behavior or API contracts.

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• cd frontend && pnpm lint

Screenshots, API examples, or logs

No screenshots or API examples. This change only reorganizes frontend admin section files and updates imports.

Configuration, migration, and compatibility notes

No configuration, migration, deployment, public API, database schema, or compatibility changes.

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including authentication-related admin UI routing and import boundaries.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.