feat: select first search result on Enter key press

[NotAbdelrahmanelsayed]

Summary

• When the cashier types a partial item name in the search box and presses Enter, the first visible search result is added to the cart immediately
• If no search results are present (e.g. typing a barcode), the existing exact barcode lookup runs as before — no regression for scanner workflows
• Variant/UOM/batch dialogs still appear when needed (respects auto-add mode)

Behaviour change

Before	After
Enter → exact barcode lookup → "Item Not Found" for partial text	Enter → first search result added to cart
Barcode scan → works as before	Barcode scan → works as before (fallback path)

Files changed

• POS/src/composables/useSearchInput.js — handleKeyDown: checks itemStore.filteredItems[0] before falling back to barcode lookup

Test plan

• Type a partial item name → results appear → press Enter → first result added to cart
• Type an exact barcode → press Enter → barcode lookup fires normally
• Type something with no matches → press Enter → "Item Not Found" warning
• With auto-add ON: Enter adds without prompts for simple items
• With auto-add OFF: Enter opens variant/UOM dialog if applicable

🤖 Generated with Claude Code

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Reopens #239, which was auto-closed by the stale-PR bot for lack of recent activity. No code changes since — recreated to bring it back into review.