Product Form Component

## Type
Feature

## Priority
🔴 High

## Description
Component for entering product details into an order:
- Product selection (dropdown or grid)
- Quantity input (decimal)
- Purchase price input (decimal)
- Sale price input (decimal)
- Real-time calculation display (margin, %)
- Buttons: Add / Cancel
- Input validation

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*Full details available in docs/tmp/github-issues-draft-en.md*
