Docs / UX: what's the difference between "queued" and "pending" messages?

## Summary
The CLI surfaces two distinct concepts — "queued" messages and "pending" messages — without explaining how they differ. From the user's perspective they look like the same thing (a message you've typed but the agent hasn't processed yet).

## Questions to resolve
- Is there a meaningful behavioral difference (lifecycle, when each is flushed, when each can be cancelled)?
- If yes: please document it in `/help` and in the on-screen labels (e.g. tooltip on the queue indicator).
- If no: please unify the terminology to a single word in the UI.

## Why this matters
Existing issues about queued-message editing/cancellation (#2055, #2378, #2905, #1857, #3692) all use "queued"; some other surfaces use "pending." Users can't tell whether they're filing duplicate reports about the same thing or two genuinely different bugs.

先比較 CLI 中對 "queued" 和 "pending" 的使用方式，接著檢查 /help 文字，以及 issue 中提到的畫面上佇列指示器標籤。判斷這些術語描述的是不同的生命週期、flush 行為，還是取消狀態；在 /help 和 tooltip 中記錄這項區別，或者如果兩者等義，則統一術語。

由索引模型根據 Issue 內容生成。