feat(tax): add 'Apply to all countries' option to tax rates dropdown

[superdav42]

Summary

PR #277 (feat(tax): add universal tax fallback feature) was closed because the maintainer requested a different implementation approach.

Maintainer's Preferred Approach

Rather than a separate toggle + rate field, add an "Apply to all countries" option to the existing country dropdown on the tax rates configuration page. This is simpler to implement and more consistent with the existing UI.

From the maintainer's review on PR #277:

"I think I'd rather implement this by adding a new option to the country drop down, 'Apply to all countries' that could be chosen on the tax rates configuration page. It would probably be simpler to implement and wouldn't need a new setting field."

Context

• Original PR: feat(tax): add universal tax fallback feature #277 (closed — wrong approach)
• Original contributor: @mahyarrezghi
• The existing tax rates page has a country dropdown — add a special "Apply to all countries" sentinel value that acts as a catch-all fallback when no country-specific rate matches

Acceptance Criteria

• "Apply to all countries" option added to the country dropdown on the tax rates configuration page
• When selected, the rate applies as a fallback for any country not explicitly configured
• No new setting fields added (uses existing tax rate row structure)
• Existing country-specific rates take precedence over the catch-all
• Translations updated for the new option string

[superdav42]

This feature is already fully implemented on main.

What's in place:

• views/taxes/list.php — "Apply to all countries" option with value="*" is the first entry in the country <select> dropdown on the Tax Rates configuration page
• inc/functions/tax.php — wu_get_applicable_tax_rates() applies rates with country='*' as a fallback when no country-specific rate matches; per-country rates always take precedence
• lang/ultimate-multisite.pot — translation string registered
• tests/WP_Ultimo/Functions/Tax_Functions_Test.php — unit tests covering wildcard fallback, precedence, and edge cases

Merged via:

• PR fix(tax): redesign universal tax as 'Apply to all countries' dropdown option #427 (dropdown + fallback logic + POT string) — closed issue fix(tax): redesign universal tax as country dropdown option per PR #277 review #426
• PR feat(tax): implement universal tax as 'Apply to all countries' option in Tax Rates page #456 (unit tests) — closed issue feat(tax): implement universal tax as 'Apply to all countries' option in Tax Rates page #455

All acceptance criteria are satisfied. Closing.