Root cause: parseCardsForPreview was called TWICE in generatePreviewHTML:
1. Line 179 - correctly parses markdown to HTML including buttons
2. Line 283 - redundantly called AGAIN after variable highlighting
After first call, variable highlighting (lines 275-280) replaced unknown
variables like {login_url} with <span>[login_url]</span>. When the second
parseCardsForPreview ran, the [login_url] text was misinterpreted as
shortcode syntax, corrupting button HTML output.
Fix: Remove the redundant second call to parseCardsForPreview at line 283.
The function is already called at line 179 before any variable replacement.
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