Fix desktop model menu height clamping

[kirafishy]

Summary

修复桌面模型选择器在小视口下高度超过可用空间、导致列表内容被裁剪的问题。

统一 vendor 菜单与 model submenu 的高度计算逻辑，并补充 trigger 上下空间和极小视口边界测试。

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

• pnpm --filter @deeix/web test
• go test ./...
• 本地前端 /chat 返回 HTTP 200
• 本地后端 /api/v1/version 返回 HTTP 200
• 登录 CORS preflight 返回 HTTP 204
• 登录接口返回 HTTP 200

Screenshots, API examples, or logs

本次为菜单布局和滚动高度修复，无需新增截图或 API 示例。

Configuration, migration, and compatibility notes

不涉及数据库迁移、API 契约或生产配置变更。

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.