Defer JSON preview parsing to avoid main-thread jank

[lynnswap]

Summary:

• WINetworkBodyPreviewData.previewData eagerly parses JSON and builds the full node tree on each access.
• WINetworkBodyPreviewView reads previewData every render, so multi-MB JSON can block the main thread even when JSON mode isn't selected.

Proposed direction:

• Cache parsed JSON nodes on WINetworkBody and only parse when JSON mode is selected.
• Parse off the main actor and publish results back on the main actor.
• Invalidate the cache when preview/full/base64 state changes.
• Use a lightweight JSON eligibility check (content-type or leading '{'/'[') to decide whether to show the JSON mode without parsing.