Add multilingual support

[PrimeBuild-pc]

Summary

• add complete Italian, French, German, Spanish, and Russian resource dictionaries alongside English and Simplified Chinese
• detect the initial language from Windows, normalize regional variants, and keep English as the safe fallback
• persist language changes reliably, including the Save action in the unsaved-settings dialog
• use neutral Windows 11-style surfaces for dialogs and status messages

Validation

• 625/625 Release tests passed
• localization dictionaries have matching keys and format placeholders
• git diff --check passed

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Codecov Report

❌ Patch coverage is 76.35135% with 35 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
Services/LocalizationService.cs	76.66%	19 Missing and 2 partials ⚠️
ViewModels/SettingsViewModel.cs	54.54%	4 Missing and 1 partial ⚠️
Services/ApplicationSettingsService.cs	90.00%	3 Missing and 1 partial ⚠️
MainWindow.Behaviors.partial.cs	0.00%	3 Missing ⚠️
Helpers/NavigationBehavior.cs	50.00%	2 Missing ⚠️

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