feat: auto-select quantity input when UOM dialog opens

[NotAbdelrahmanelsayed]

Summary

• Add ref="quantityInput" to the quantity <input> in ItemSelectionDialog
• Import nextTick from Vue
• When the dialog opens in uom mode, automatically focus and select the quantity field so users can immediately type a new quantity without manually clearing the default value

Behaviour Before / After

Before	After
Dialog opens, cursor is not in the quantity field	Dialog opens, quantity field is focused and its value is selected — type to overwrite instantly

Testing

1. Open POS and add an item that has multiple UOMs configured.
2. Click the item to open the UOM selection dialog.
3. Verify the quantity input is focused and the default value (1) is selected.
4. Type a new quantity — it should replace the selected value immediately.

[NotAbdelrahmanelsayed]

auto-select.mp4