On export, card margin of `0` still renders a gap; only negative values remove it

[jonborchardt]

Summary
Setting card margin to 0 still leaves a visible gap. Using -0.01 removes the gap, implying layout rounding or padding.

Steps to Reproduce

1. Create two cards.
2. Set margin to 0.
3. Render/preview/export.
4. Observe gap between cards.
5. Set margin to -0.01 and re-render.

Expected

• Margin 0 yields no visible gap.

Actual

• A small gap persists at 0; negative margin removes it.

Acceptance Criteria

• With margin 0, there is no visible gap at 100% zoom and in export.
• Behavior consistent across DPI/zoom values.
• No need for negative margins to remove gutters.