Improve cashier layout and touch scrolling and fix categories column width

[IanTheMitten]

Motivation

• Resolve overflow and scrolling issues in the cashier UI so panels and lists scroll correctly across viewports, including touch devices.
• Ensure the category column width is appropriate and avoid layout overflow caused by an oversized sidebar.

Description

• Updated CartSection.tsx to make the items list scrollable with min-h-0, overflow-y-auto, overscroll-contain, and touch-pan-y, and reset paymentMode after completion.
• Updated CashierPage.tsx container and side panels to include min-h-0 to prevent flex overflow and ensure proper child scrolling behavior.
• Updated ProductSearch.tsx to constrain the categories column width from 880px to 280px, add touch-friendly scrolling with min-h-0, overflow-y-auto, overscroll-contain, touch-pan-y, and style={{ WebkitOverflowScrolling: 'touch' }} on scrollable panes.

Testing

• Ran the project test suite with yarn test and linting with yarn lint, and they completed successfully.
• Performed a local UI smoke check to verify scrolling and layout behavior on desktop and touch emulation, and the changes behaved as expected.

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Codex Task