Add POS dialog components in Shared/Dialogs/:
- VoidRefund.razor: Void/refund dialog with order lookup, type selection, reason, manager PIN
- OrderEdit.razor: Edit existing order with qty controls, add items, discount, notes
- OrderCancel.razor: Cancel order confirmation with reason dropdown, refund warning
- SplitBill.razor: Split bill with equal/by-item/custom modes
- StockIn.razor: Quick stock-in with supplier, lot, expiry, recent log
- StockOut.razor: Stock-out with current stock display, reason selection
- StockTransfer.razor: Transfer between branches with product list
- PriceCheck.razor: Price check with hero price, promotions, price history
All files use @layout PosLayout, @inherits PosBase, bilingual comments,
CSS variables, Vietnamese demo data with VND prices.
Co-authored-by: Velik <hongochai10@users.noreply.github.com>
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