fix(editor): preserve code languages in Live imports

[rclod]

Description

Plane Live now preserves code-block language metadata when it imports HTML into Yjs. The server-side DOM implementation does not populate firstElementChild as the browser does, so language-* classes were dropped during the first collaborative-editor load even when the submitted HTML was correct.

This is useful for every syntax-highlighted code block and lets the Mermaid renderer in #9787 recognize agent-authored language-mermaid blocks after Live conversion.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Improvement (change that would cause existing functionality to not work as expected)
• Code refactoring
• Performance improvements
• Documentation update

Screenshots and Media (if applicable)

Not applicable; this fixes server-side document conversion.

Test Scenarios

• Added an HTML → Yjs → JSON/HTML regression test for language-mermaid.
• pnpm turbo run build --filter=live (10/10 tasks).
• Live tests (3 files, 33 tests), type-check, lint (6 existing warnings, 0 errors), and format checks.
• Deployed the same change on a Plane CE v1.4.2 instance and verified production Live conversion preserves language="mermaid" for state, flowchart, and sequence diagrams.

References

Related: #9787

[coldtea-pr-lens]

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🟢 +0 new · 🟠 ~2 changed · 🔴 -0 removed · 1 flow · 2 files · commit b448751

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2 components touched across 2 lanes.

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Data flow

Parsing code block language from HTML

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