P3: Add scroll affordance hint on mobile category tabs

[hasitpbhatt]

Problem: On mobile, categories overflow with hidden scrollbar (Apple-style) but there's no visual hint that more categories exist beyond the edge. Safari uses a subtle shadow/gradient edge fade.

Fix:

• Add CSS mask-image with a linear-gradient on the .categories container to create a fade edge on the right side
• The gradient shifts as the user scrolls to reveal the edge

Files to modify:

• css/style.css — .categories and .filter-chips mobile styles