fix(desktop): hide single Host settings selector

[M4n5ter]

English

Summary

Hide the Runtime Host selector in Settings when only one Host is enabled. The selector remains visible when multiple enabled Hosts provide a real choice, including enabled Hosts that are temporarily unavailable.

Verification

• Desktop preload, main, renderer, and Storybook typechecks passed
• Repository formatting and focused Biome lint passed
• git diff --check passed
• Verified before and after in the same real Electron window with an isolated Local-only profile through agent-browser

Screenshots

Before	After

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex implemented the condition and performed type, formatting, lint, and real-window verification under maintainer direction

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

简体中文

摘要

当 Settings 中只有一个已启用 Host 时隐藏 Runtime Host selector。存在多个已启用 Host、因而确实可选择时仍显示 selector，包括暂时不可用但仍处于启用状态的 Host。

验证

• Desktop preload、main、renderer 与 Storybook typecheck 通过
• 仓库格式检查与聚焦的 Biome lint 通过
• git diff --check 通过
• 使用隔离的纯 Local profile，通过 agent-browser 在同一个真实 Electron 窗口完成 before/after 验证

截图

修改前	修改后

AI 使用

• 没有生成式工具作出实质性贡献
• 生成式工具作出了实质性贡献

工具与范围：OpenAI Codex 在维护者指导下实现条件修改，并完成类型、格式、lint 与真实窗口验证

检查清单

• 测试覆盖本次变更，并会在缺少本次变更时失败
• lint、格式检查、类型检查和受影响检查均在本地通过

本 PR 是否会改变行为？

• 是 — 已在上方摘要中说明
• 否

[Astro-Han]

LGTM.