Fix compare-mode prompt templating preview

[MariaHov]

Summary

• In compare mode, show full-prompt previews for both Prompt A and Prompt B per case
• Mark case fields that appear in only one template with small “A only” / “B only” notes
• Re-render case fields when compare mode is toggled so placeholders stay in sync

Test plan

• Enable features.promptTemplating and allowCompare
• Use different {{placeholders}} in Prompt A vs B and confirm union of fields + usage notes
• Confirm dual full-prompt preview updates when case field values change
• Toggle compare off and confirm single-preview UI returns

Made with Cursor

[coderabbitai]

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