fix(import): keep the content a compound submit button wraps

[tommy230]

Summary

A submit button that carries an icon instead of text loses the icon on HTML import. A newsletter form whose submit is an arrow drawn as an inline <svg>, a search box whose submit is an icon-font <i>, or a form whose submit is an <img> all import as base.submit with only the 'Submit' fallback label, and the published page shows that word where the icon was.

<!-- source -->
<form><button type="submit"><svg viewBox="0 0 13 12"><path d="M7.2 11.7 12.2 6.7"/></svg></button></form>

<!-- published today -->
<form><button type="submit">Submit</button></form>

This is the case #476 deliberately left out. base.submit in src/modules/base/forms/index.ts is a leaf whose only content is the label prop, so the button rule in src/core/htmlImport/rules.ts does not recurse into submit buttons and submitLabel() falls back to 'Submit'. The comment there notes that lifting this needs base.submit to accept children; this change does that.

The fix:

• base.submit accepts children and renders them when it has any, falling back to label only when it has none. The rule lives in src/modules/base/forms/content.ts and is shared by the publisher render and the canvas preview, the same shape base.link uses.
• The importer recurses into submit buttons with element children, so an inline <svg>, an icon-font <i>, or an <img> survives as child nodes. Text-only submits still import as a childless base.submit with their label, and <input type="submit"> is unchanged.
• Submit buttons stay base.submit rather than becoming a container, so core/forms still finds the form's submit control by module id.

Trade-off: the Form Settings panel and the form snapshot still read the submit's label prop, so an icon-only submit shows as "Submit" there even though the canvas and the published page show the icon. That is cosmetic and left alone here.

Overlaps with #575 on adjacent docs/features/html-import.md bullets; whichever lands second gets a small rebase.

Verification

• bun run build
• bun run lint
• bun test: 7061 pass, 0 fail
• Docker/deployment check, if relevant: not relevant

New tests in src/__tests__/htmlImport/submitButtonChildren.test.ts: 6 of 8 fail on main and pass with this change; the two that pass either way cover the unchanged text-only and label-fallback cases. The existing structurePreservation test that pinned the old label-only behaviour now asserts the children are kept.

Checklist

• Tests cover behavior changes.
• Docs were updated when behavior, config, deployment, or public surfaces changed. (docs/features/html-import.md: the button row and the "Button-like elements keep what they wrap" note; docs/features/modules.md and docs/reference/module-engine.md.)
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.

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