polish(settings): simplify models section chrome and clarify Subagent default copy

[bobleer]

Summary

• Remove nested section body chrome on the AI Models settings page so provider-group cards are the only visual containers.
• Clarify zh-CN / zh-TW copy for the Subagent default model description.

Test plan

• Open Settings → AI Models and confirm the models section no longer shows an extra bordered panel around provider groups.
• Switch to zh-CN / zh-TW and confirm the Subagent default model description reads clearly.
• pnpm run type-check:web
• pnpm run i18n:audit