[建议 / Feature] Turns fail silently when pasted images exceed ~4MB total — please auto-compress and show the budget before sending

提交前确认 · Pre-submission checklist

• 我已搜索过现有 issue，确认这不是重复提议 / I searched existing issues and confirmed this isn't a duplicate.
• 我已阅读 CONTRIBUTING.md / I've read CONTRIBUTING.md.

问题类别 · Category

UI / 界面体验 · UI / UX

涉及的 Agent 框架 · Agent framework

ZCode Agent（自研）

使用场景 · Use case

When the total size of image attachments in one message exceeds ~4MB, the entire turn is rejected at input processing. No tools run, nothing is executed, and the chat shows only a generic failure — it looks like the agent "did nothing" rather than an attachment-size problem. Full-res screenshots (the norm in web-UI testing workflows) hit this constantly.

建议方案 · Proposal

Expected behavior
Pasted images are automatically downscaled/re-encoded client-side to fit the budget (this is what comparable agent GUIs do — paste-and-send just works).
If auto-compression can't solve it, the composer shows the size budget BEFORE sending, not a dead turn after.

预期价值 · Expected value

No response

你认为的优先级 · Your perceived priority

中 · Medium

你使用的 ZCode 版本 / 环境 · ZCode version / environment

ZCode 3.7.7.4926 / Desktop / Win 11

补充材料 · Additional context

Environment
ZCode desktop, Windows 11
Model: GLM-5.3 (Z.ai coding plan)
Trigger: pasting/attaching multiple full-resolution screenshots in one message

Steps to reproduce
Start a session and paste 3–4 full-res screenshots (e.g. 2560×1440 PNGs, ~4MB each).
Send the message.
The turn fails immediately. No tool call is ever executed.

Start with the reported reproduction: paste 3–4 full-resolution screenshots into a ZCode desktop session on Windows 11 and send them. The issue does not name source files, tests, or entry points. Done means the client prevents silent failure by compressing attachments or showing the size budget before sending.