Improve mobile responsiveness on POS page

## Description

The Point of Sale page (`src/app/c/[slug]/pos/`) needs better mobile and tablet responsiveness for use on smaller screens and handheld devices. ## Current Behavior

The POS layout is optimized for desktop screens. On mobile devices, the item grid and cart panel overlap or require excessive scrolling. ## Expected Behavior

• Item grid should stack vertically on mobile
• Cart should be accessible via a slide-up panel or toggle button
• Category tabs should be horizontally scrollable

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