raw-apps: prevent + surface the silent blank-screen (unmounted #root)

[Guilhem-lm]

Summary

AI-generated (or hand-written) React raw apps could render a blank screen with no error when index.tsx was written as a bare export default function App() {...} that never mounts anything. The preview harness bundles and executes index.tsx against an empty <div id="root"> but does not auto-render a default export — the entrypoint must itself call createRoot(document.getElementById('root')!).render(<App />). When it doesn't, the JSX lives inside an uncalled function, so nothing throws and the runtime-error overlay stays silent.

This PR tackles the failure mode from two sides:

1. Prevent it — strengthen the raw-app AI prompt so the mount requirement is explicit.
2. Surface it — when a build still renders nothing, show an info overlay in the editor that names the exact fix.

Changes

Prompt (prevention)

• system_prompts/base/raw-app.md — rewrote the Entrypoint section: index.tsx is the bundling and mount entrypoint, MUST mount App into #root (React/Svelte/Vue snippets), and never be replaced with a bare component. Added a blank-screen debugging note.
• frontend/src/lib/components/copilot/chat/app/core.ts — same mount-entrypoint guidance in the in-chat app system prompt's Frontend section.
• Regenerated derived files via system_prompts/generate.py: prompts.ts, skills/raw-app/SKILL.md, cli/src/guidance/skills.gen.ts.

Overlay (detection)

• frontend/src/lib/components/raw_apps/RawAppEditor.svelte — handle a new emptyRender message from the preview iframe and render an info Alert ("Nothing rendered") pointing at the missing createRoot(...).render(...) call. Mirrors the existing runtime-error overlay; cleared on next build.
• Also handle renderAppeared — the preview retracts the hint if a slow app (async fetch, Suspense) mounts #root after the grace window, so a legitimate late render can't leave a false-positive overlay.
• Both messages are emitted by the preview harness in the builder repo: windmill-labs/windmill-code-ui-builder#17. That change polls #root after each build and posts emptyRender when nothing mounted (suppressed if a runtime error already explained the blank screen), then watches for a late mount to post renderAppeared.
• The host handlers here are dormant-safe until the builder tarball that emits these messages is pinned, so they land ahead of the artifact bump.

Screenshots

A bare-component index.tsx (export default App, no createRoot) builds successfully (see logs) yet mounts nothing — the "Nothing rendered" overlay names the fix.

Sequencing

The overlay only fires once the builder tarball emits emptyRender:

1. Merge builder PR windmill-labs/windmill-code-ui-builder#17 → CI publishes a new tarball to R2.
2. Bump frontend/scripts/ui_builder_artifact.json (new version + sha256) — follow-up commit on this PR.

Test plan

• Generate a React raw app via copilot chat; confirm index.tsx includes createRoot(document.getElementById('root')!).render(<App />) rather than a bare component.
• Confirm source↔generated consistency (grep the new wording in all three generated files).
• Bare-component build shows the "Nothing rendered" overlay; overlay clears on a rebuild that mounts #root; runtime error suppresses it (browser-verified, see screenshot).
• False-positive retraction (browser-verified against the running editor): late-mount build (#root filled at 1000ms) → overlay appears @612ms, retracts @1018ms via renderAppeared; permanently-empty build keeps the overlay; immediate render never shows it.

🤖 Generated with Claude Code

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