fix: allow deleting first time slot in availability schedule

[aminjaoui]

Fixes #26448

Previously, users could not delete the first time slot in their availability schedule. The trash icon only appeared on slots 2+, while the '+' button only appeared on the first slot.

Changes:

• Show '+' button on all time slots for easier adding
• Show delete button on all slots when there are 2+ slots
• Prevent deletion when only 1 slot remains (must have at least one)

This improves UX by allowing users to remove any time slot they want, not just the ones after the first.

What does this PR do?

Allows users to delete the first time slot in their availability schedule when multiple slots exist.

• Fixes Calendar override remove first time #26448

Demo (For contributors especially)

Before:

• Slots 2 and 3 have delete (trash) icons
• Slot 1 has NO delete icon, only a '+' button

After:

• All slots have both '+' and delete icons when there are 2+ slots
• Cannot delete when only 1 slot remains

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - no docs changes needed.
• I confirm automated tests are in place that prove my fix is effective or that my feature works. Existing tests cover the component; manual testing performed.

How should this be tested?

1. Go to Settings > Availability
2. Select any schedule
3. Add multiple time slots (e.g., 3 slots) for any day
4. Verify ALL slots now show delete buttons
5. Delete the first slot - should work
6. Verify you cannot delete when only 1 slot remains

No special environment variables needed.

Checklist

• I have read the contributing guide
• My code follows the style guidelines of this project
• N/A - simple conditional change, no complex logic needing comments
• My changes generate no new warnings

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[cubic-dev-ai]

No issues found across 1 file

[KRUPANIDHIBEHERA]

From a product and UX perspective, this change makes sense.

Allowing users to delete any time slot improves flexibility and reduces
confusion during availability setup. The guardrail of preventing deletion
when only one slot remains is a good balance between user control and
avoiding invalid states.