fix(frontend): block deletion of app's last revision

[JasonOA888]

Problem

A user can delete all revisions of an app from the Playground. Opening Playground after that leaves it in a broken state with no explanation.

Solution

• Add isLastRevision check that counts total visible revisions across all variants
• Disable delete button when deleting would remove all revisions
• Show helpful message: Cannot delete the only revision. Delete the app instead.

Changes

• Modified DeleteVariantModal/Content.tsx
• Added logic to detect when deletion would remove the last revision
• Button is disabled with tooltip explaining why
• User sees clear message about what to do instead

Testing

1. Create an app with a single revision
2. Open Playground and try to delete via header menu (⋯ → Delete)
3. Expected: Delete button is disabled with message

Fixes #3892

[vercel]

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[mmabrouk]

Thank you @JasonOA888 for the PR. }

Can you please share a video demo of the implemented feature?

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[JasonOA888]

@mmabrouk Thanks for reviewing!

I don't have a local Agenta development environment set up to record a video demo. However, I can walk through the logic:

What the PR Does

1. Detects last revision: Counts total visible revisions across all variants
2. Disables delete button: When isLastRevision is true
3. Shows user guidance: "Cannot delete the only revision. Delete the app instead."

Code Changes

// New logic to detect last revision
const isLastRevision = useMemo(() => {
  let totalRevisions = 0
  Object.values(variantGroups).forEach((group) => {
    totalRevisions += group.totalIds.length
  })
  return totalRevisions > 0 && totalSelectedCount >= totalRevisions
}, [variantGroups, totalSelectedCount])

// Disable button and show message
<Button disabled={... || isLastRevision} ... />
{isLastRevision && <Text>Cannot delete the only revision...</Text>}

Testing Steps (for maintainer)

1. Create an app with a single revision
2. Go to Playground, try to delete via ⋯ menu
3. Expected: Delete button disabled, message shown

Would a code walkthrough or screenshots be acceptable instead? I can set up a local environment if video is mandatory.