ROOT CAUSE: Complete flow trace revealed syntax mismatch:
- blocksToMarkdown outputs NEW syntax: [card:type], [button:style](url)Text[/button]
- markdownToBlocks ONLY parsed OLD syntax: [card type="..."], [button url="..."]
This caused buttons/cards to be lost when:
1. User adds button in Visual mode
2. blocksToMarkdown converts to [button:solid]({url})Text[/button]
3. handleBlocksChange stores this in markdownContent
4. When switching tabs/previewing, markdownToBlocks runs
5. It FAILED to parse new syntax, buttons disappear!
FIX: Added handlers for NEW syntax in markdownToBlocks (converter.ts):
- [card:type]...[/card] pattern (before old syntax)
- [button:style](url)Text[/button] pattern (before old syntax)
Now both syntaxes work correctly in round-trip conversion.
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