Fix ISO week calculation in backup rotation policy

Replace manual ISO week calculation with System.Globalization.ISOWeek API.
The manual calculation had bugs around year boundaries where thursday.DayOfYear
and jan4.DayOfYear could be from different years, causing incorrect week numbers.

This fixes the BackupRotationPolicyTests.ApplyRotationAsync_KeepsWeeklyBackups test
which was failing when backups crossed year boundaries (Dec 2025 -> Jan 2026).

Author: nickprotop

LocalizationManager.Core/Backup/BackupRotationPolicy.cs

@@ -164,18 +164,9 @@ public async Task ApplyRotationAsync(BackupManifest manifest, string backupDir)
     /// </summary>
     private string GetIsoWeekKey(DateTime date)
     {
-        // ISO 8601 week number
-        var dayOfWeek = (int)date.DayOfWeek;
-        var dayOfYear = date.DayOfYear;
-
-        // Adjust for ISO 8601: Monday = 1, Sunday = 7
-        var isoWeekDay = dayOfWeek == 0 ? 7 : dayOfWeek;
-
-        // Find the ISO week number
-        var thursday = date.AddDays(4 - isoWeekDay);
-        var isoYear = thursday.Year;
-        var jan4 = new DateTime(isoYear, 1, 4);
-        var isoWeek = (thursday.DayOfYear - jan4.DayOfYear) / 7 + 1;
+        // Use built-in ISO 8601 week calculation
+        var isoYear = System.Globalization.ISOWeek.GetYear(date);
+        var isoWeek = System.Globalization.ISOWeek.GetWeekOfYear(date);
 
         return $"{isoYear}-W{isoWeek:D2}";
     }