[FEATURE] Desktop: "screenshot / mockup → working code" workflow with built-in live preview (like CUE / code-to-UI)

Feature request

GitHub/Codex CLI have agentic PR review loops; Claude Code desktop has visual diff; Trae has entire "screenshot to UI code". OpenCode currently does not have a first-class "image/screenshot → working UI code" workflow in the desktop app.

What I'd like

In the Desktop app, a flow like Trae's "CUE" / Gemini CLI's "Design to Code":

1. User drags a screenshot (or picks a clip) into the chat → the image is attached (already works via drag & drop).
2. A dedicated workflow (e.g. /screenshot-code or a "convert to UI" button) tells the model:
   • generate a complete single-file HTML/CSS/JS (or based on project's framework: React/Vue) implementing that mock,
   • plus a running preview (live). Desktop currently has no way to preview a rendered result.

Why

• Many users use opencode desktop to build UIs; "截图转页面" is the single most requested productivity flow in async (Chinese) forums, and is the biggest Trae/Claude difference for UI tasks.
• There's currently no built-in preview: we can only open an external browser and commit changes.

Related

• #906 (paste to attach image) — that's the input side, works. This request is about the workflow side: next step generated code + live preview inside the desktop app.

Scope suggestion

• MVP: detect image attachments → ask model to generate a single self-contained index.html prototype → open in embedded webview (desktop has webui already) with hot-reload on file save.
• Later: framework-aware generation (next/src/app) and diff apply.

This is a quick win: the tools exist (model can read images; desktop has webview), only the UX workflow is missing.