Product Variant Tree View Qty Available

This module makes it quicker to inspect where a product variant's stock actually is. In the Product Variants list view, an 'Open' button now appears right after the on-hand quantity column. Clicking it opens the stock quant view filtered for that variant, so users can see the locations, lots and quantities behind the number.

The button is only shown for variants whose product type is 'product' (storable goods); it is hidden for services and other types. The action is named after the variant, for example 'My Product - Update Quantity', so it is clear which product is being inspected.

The module also adjusts how the 'on hand' filter works on stock quants. When users filter quants by on-hand status, the search now respects the locations the current user is allowed to see, instead of using a fixed rule. Unsupported filter combinations raise a clear error message rather than returning wrong results.

It touches the product.product and stock.quant models and extends the existing product variant tree view. No new menus are added; the entry point is the button inside the Product Variants list.