feat: add file drag and drop functionality to chat input

[chenyme]

Summary

Add drag-and-drop attachment upload support in the chat page. Dropped files reuse the existing chat attachment upload flow, while the composer shows a minimal drop state through its existing input styling and placeholder.

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
• cd frontend && pnpm build
• git diff --check

Screenshots, API examples, or logs

Not included. UI change is limited to chat composer drag-over/drop state.

Configuration, migration, and compatibility notes

No configuration, API, database migration, or deployment changes.

Existing file upload policy, attachment limits, validation, and upload behavior are reused. Existing click-to-upload and paste-to-upload behavior remains unchanged.

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, authorization, provider routing, file processing, billing, and admin APIs where relevant.

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.