Hard corners and more margin on card elements

Card elements (.card, .task-card, .stat-card, etc.) have rounded corners (border-radius: 6px) and very little margin between them, reducing readability.

Fix: 1. Remove border-radius from cards — set to 0 or 1px for hard corners 2. Add more margin between cards for visual breathing room (currently near-zero, increase to 8-12px)

Implementation:

• In Omni/Web/Style.hs: change radiusSm, radiusMd, radiusLg to 0 (or just set card border-radius to 0)
• Actually, only change card-specific radius. Keep radius on badges and small interactive elements.
• In Omni/Task/Web/Style.hs cardStyles: set borderRadius to 0, increase margin
• In Omni/Web/Style.hs: update any card-related margin/gap styles
• Check .list-group, .task-card, .stat-card, .card gap/margin values

Key files: Omni/Web/Style.hs, Omni/Task/Web/Style.hs Namespace: Omni/Task.hs