editor: the viewer scene header's back arrow is optional

[AxiomeCG]

What does this PR do?

ViewerSceneHeader can show no back arrow: backHref: null hides it. An embedded viewer (/viewer/<id>?embed=1 in pascalorg/private-editor#838, which renders the editor's Preview in a listing's iframe) has nowhere to go back to unless the embedding page gives a returnTo. Existing callers pass a string or nothing, so their arrow is unchanged.

How to test

1. Every current use renders as before: Preview's header still has its back button (onBack), and the public viewer's header still links back (backHref).
2. With pascalorg/private-editor#838, /viewer/demo_1?embed=1 shows the header with no back arrow, and /viewer/demo_1?embed=1&returnTo=/community shows one linking to /community (its viewer-embed.spec.ts covers both).

Screenshots / screen recording

N/A: the visible change is in #838's embed.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch

🤖 Generated with Claude Code

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Note

Low Risk
Small conditional UI in the viewer header with no auth or data changes; default back link behavior is preserved.

Overview
ViewerSceneHeader now treats backHref: null as “no back control”: when there is no onBack handler, the header skips the back Link instead of defaulting to /.

The prop type is string | null and the docs call out embedded viewers (e.g. iframe) that may have no sensible return URL unless the parent passes one.

Existing behavior is unchanged for callers that omit backHref (still /) or pass a string; onBack still wins over backHref.

^{Reviewed by Cursor Bugbot for commit a18817c. Bugbot is set up for automated code reviews on this repo. Configure here.}

[pascal]

This PR makes the back arrow in ViewerSceneHeader optional. backHref now accepts string | null, and passing null renders no arrow at all. The default stays '/', so callers that pass a string or omit the prop get the same header as before, and onBack still wins when it's supplied. The motivating case, per the description, is the embedded viewer in pascalorg/private-editor#838 (/viewer/<id>?embed=1), which has nowhere to go back to unless the embedding page supplies a returnTo.

File	Change	What changed
packages/editor/src/components/viewer/viewer-scene-header.tsx	modified	backHref widened to string | null, doc comment updated, and the render branch returns null instead of the Link when backHref === null

Start with the render branch around line 111: the new backHref === null ? null : arm is the whole behavior change, and the rest is the type and comment that go with it.