feat: remove model header constant in chat model picker

[chenyme]

Summary

Fix several frontend UI consistency issues in the chat composer and model picker.

• Removed the title/header area from the desktop model picker secondary menu to avoid confusing it with selectable model rows.
• Updated the secondary model menu height calculation after removing the header so scrolling and viewport collision handling remain correct.
• Fixed input placeholder font sizing by letting Input and Textarea placeholders inherit the actual input font size instead of forcing text-xs.

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
• git diff --check

Screenshots, API examples, or logs

UI-only change. No API examples or backend logs.

Configuration, migration, and compatibility notes

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