polish(web): inset compose criteria empty state from the card divider

[AlexsanderHamir]

Summary

• The empty done-criteria placeholder sat flush against the card divider (8px top, 0 sides/bottom), so the dashed box collided with the hairline above it.
• Inset .task-create-checklist-empty with --space-4 / --space-6 so it breathes inside the card. Filled rows stay full-bleed.

Test plan

• Open task compose with no done criteria and confirm the dashed empty state is clearly separated from the divider and card edges.
• Add a criterion and confirm the filled list still spans the card width with row separators unchanged.
• Remove the last criterion and confirm the empty state inset returns.

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