refactor: update chat model picker with new popover component

[chenyme]

Summary

Refactor the chat model picker to improve mobile pricing access and desktop multi-provider navigation.

The model picker now uses a controlled popover with a floating model panel, so provider and model lists can scroll independently, stay inside the viewport, and avoid clipping near screen edges. The pricing $ action is separated from model selection to prevent mobile taps from immediately selecting/closing the model menu. Also normalized stream abort handling and fixed mobile sidebar collapsed-state behavior.

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
• cd frontend && pnpm build; interrupted manually while waiting on Creating an optimized production build ...

Screenshots, API examples, or logs

UI-only change. No API examples or logs are required.

Configuration, migration, and compatibility notes

No configuration, database migration, environment variable, Docker, 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.