Change vat management for supplier prices

[SylvainLegrand]

When creating a supplier price:
1 - The default VAT rate is the product's sales VAT rate. In many situations, this isn't a problem because the sales and purchase VAT rates are the same. However, for the construction sector, with its reduced sales VAT rate, this behavior is inconsistent.
2 - If the multi-currency function is enabled, no default currency is offered, which is a waste of time in 80% of cases.

This fix applies the default purchase VAT rate and the company's base currency.

[Copilot]

Pull request overview

This PR adjusts the default values proposed when creating/editing supplier prices to better match purchasing workflows (notably in cases where sales VAT differs from purchase VAT, and when multicurrency is enabled).

Changes:

• Change the suggested default supplier VAT from product sales VAT to a standard country VAT rate (by not passing the product id into get_default_tva()).
• Always default supplier price currency to the company base currency when no currency is set.
• Default the multicurrency rate display fallback to 1.

Comments suppressed due to low confidence (2)

htdocs/product/price_suppliers.php:642

• The VAT default can legitimately be 0 (e.g., seller not subject to VAT). With the current logic, a 0 result from get_default_tva() will later be treated as “empty” (notably via the $default_vat != '' check when building $vattosuggest), so the field renders blank and the form validation then rejects the submission. Consider using a strict empty-string check (or is_numeric) when deciding whether to format/show the default VAT so that 0 is preserved and displayed as "0".

			$default_vat = get_default_tva($mysoc2, $mysoc, 0, 0); // use standard country VAT rate, not product sale VAT rate (accounting non-sense for purchases)
			$default_npr = get_default_npr($mysoc2, $mysoc, 0, 0);
			if (empty($default_vat)) {
				$default_npr = $default_vat;
			}

htdocs/product/price_suppliers.php:706

• The currency rate <input> value is built by printing GETPOST('multicurrency_tx') and then printing the formatted $vatratetoshow, which can concatenate two values when the form is re-rendered after a POST (e.g., "0.50.5"), producing an invalid value. Build the value from a single computed source (e.g., $vatratetoshow) and use GETPOSTISSET('multicurrency_tx') to distinguish an explicitly-posted empty string from “not provided”.

				print '<input class="flat width50" name="multicurrency_tx" value="';
				print GETPOST('multicurrency_tx');
				$vatratetoshow = GETPOST('multicurrency_tx') ? GETPOST('multicurrency_tx') : (isset($object->fourn_multicurrency_tx) ? $object->fourn_multicurrency_tx : '1'); // change: default rate to 1
				if ($vatratetoshow !== '') {
					print vatrate($vatratetoshow);
				}
				print '">';

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[Copilot]

Typo in the inline comment: "non-sense" is typically spelled "nonsense". Consider also rephrasing to a more neutral description (e.g., "not appropriate for purchases").

Suggested change

	$default_vat = get_default_tva($mysoc2, $mysoc, 0, 0); // use standard country VAT rate, not product sale VAT rate (accounting non-sense for purchases)
	$default_vat = get_default_tva($mysoc2, $mysoc, 0, 0); // Use standard country VAT rate, not product sale VAT rate (not appropriate for purchases)

[eldy]

The default vat rate of a product depends on 3 param: the country seller, the country buyer and the product itself. Whatever ir country, the rule dependson this 3 parameters so there is no reason to remove the of of project.
For exemple if default vat for product X when seller is german is 10% , we must get 10%,but for other products like product Y, the default vat rate may be 12%.

If there is this rule to define a default vat rate when you sell as a french to a german, you should have exactly the same rule when you buy as a german to a french company.
Becasue selling product X from FR to DE is sameas buying product X from DE to FR

It will be easier to describe how to reproduce the bug with use cases of values you set, you get and you expect to get.