feat: Add data-nesting-level to HTML export

[YousefED]

Summary

Adds a data-nesting-level attribute to the flattened HTML export of blocks, preserving nesting metadata.

Rationale

Previously, nested blocks were flattened in the HTML export to keep the output structure simple. However, this meant that indentation information was lost for non-list blocks.
With this change, blocks are still flattened in the export (maintaining simplicity), but we now include a data-nesting-level attribute. This metadata allows consumers to apply appropriate styling (e.g., margins) or process the nesting structure without needing a complex nested DOM structure.

Changes

• Modified serializeBlocksExternalHTML.ts to track nestingLevel during recursive serialization.
• Added data-nesting-level attribute to the output HTML element if nestingLevel > 0.
• Updated snapshots to reflect the new attribute.

Impact

• HTML Export: The exported HTML will now include data-nesting-level attributes on nested blocks. This is a non-breaking change but affects the output HTML string.

Testing

• Automated Tests:
  • Existing snapshots for HTML export have been updated to include the new attribute.
  • Verified via pnpm test (snapshots updated by user).

Checklist

• Code follows the project's coding standards.
• Unit tests covering the new feature have been added. (Snapshot tests updated)
• All existing tests pass.
• The documentation has been updated to reflect the new feature (N/A - internal serialization change)

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Project	Deployment	Review	Updated (UTC)
blocknote	Ready	Preview	Jan 2, 2026 6:28pm
blocknote-website	Ready	Preview	Jan 2, 2026 6:28pm

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commit: 5f9fd91