Add keyboard shortcuts for POS operations

## Description

Add keyboard shortcuts to the POS page for faster cashier operations. ## Suggested Shortcuts

• \`F1\` — Focus search/barcode input
• \`F2\` — Open customer selector
• \`F5\` — Clear cart
• \`F8\` — Hold order
• \`F12\` or \`Enter\` — Pay / Complete sale
• \`Esc\` — Cancel / Close modal
• \`+\` / \`-\` — Increase/decrease quantity of selected item ## Implementation
• Add a \`useHotkeys\` hook or use an existing library like \`react-hotkeys-hook\`
• Show a keyboard shortcut help ove