Fix: Item: Several issues fixed in new item UI

The following fixes were added in the new item UI:

In the new Item UI, suppliers were previously fetched multiple times across the Item Overview, Item Details, and Variation Details tabs, leading to redundant loading and inconsistent performance. We refactored these areas to use a searchable select component for supplier selection, ensuring a single, consistent loading strategy wherever suppliers are displayed.
When the Item UI version 2 was hidden from the Setup » Item » Item UI menu, the Item roles were not visible in the Roles details view.
Newly created items displayed “Name missing for language DE” in the variation view when opened immediately after creation.
In the Order Properties widget the surcharge column was missing from the table.
In the Multichannel tab where switching between variations incorrectly retained sales channels from the previously selected variation.
In the Item Overview where variation tooltips were cut off when a large number of tags were linked.

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