Fix code review issues: factory calls, debug statements, and type hints

[Copilot]

Addresses 8 code review findings from PR #282 covering incorrect factory usage, debug statements left in production code, missing test attributes, and type safety improvements.

Factory Fixes

• InvoiceFactory.php - Changed Product::factory() to TaxRate::factory() when creating tax rates
• ExpenseItemFactory.php - Replaced 4 dd('die early') statements with proper factory fallbacks for Product, ProductUnit, and TaxRate creation

Test Consistency

• UsersTest.php - Added missing #[Test] and #[Group('crud')] attributes to it_deletes_a_user method

Code Quality

• NumberFormatter.php - Changed mb_rtrim() to rtrim() for ASCII character trimming (better performance)
• AbstractAdminPanelTestCase.php - Added ?User type hint to $superAdmin property

// Before: Debug statements halt execution
if (!$taxRate) {
    dd('die early');
}

// After: Factory creates missing records on-the-fly
if (!$taxRate) {
    $taxRate = TaxRate::factory()
        ->state(['company_id' => $company->id])
        ->create();
}

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