feat: enhance media handling and improving text streaming experience

[chenyme]

Summary

Fix chat message scrolling behavior and tighten user-message action tooltip labels.

The chat viewport scroll controller was refactored into a single follow/manual policy so streaming updates, resize events, manual scrolling, and the “scroll to bottom” action no longer fight each other. User messages now use concise tooltip labels for retry, edit, and copy actions.

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

No API changes.

Manual browser verification was not completed because the local dev server port bind was blocked in the sandbox environment.

Configuration, migration, and compatibility notes

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

Existing conversations and message data remain compatible.

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.