fix(settings): simplify model pool empty state copy

[GCWing]

Summary

Shorten the empty-state placeholder shared by the four model-pool selectors to “暂无可用模型”, with matching concise English and Traditional Chinese translations.

Type and Areas

Type: UI/UX fix

Areas: Web UI, i18n resources

Motivation / Impact

When no models are configured, each selector repeats setup instructions that are too long for the field. The placeholder now states only that no models are available.

Before: 暂无可用模型，请先添加 API 模型或导入订阅服务

After: 暂无可用模型

Verification

• pnpm run i18n:audit — passed with 0 warnings.
• git diff --check — passed.
• Resource-only copy change. Browser/computer-use verification was skipped per the user's repository instructions.
• Remote workspace, remote control, Peer Device Mode, and Detached Dispatch were not exercised; runtime, protocol, and persistence behavior are unchanged.

Reviewer Notes

AI-assisted change. Testing level: lightly tested (i18n audit and diff check). Only three existing translation values changed.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.