Add Simplified Chinese localization and language settings

[Jill-Stingray-L]

Summary

• extract user-facing text into standard .NET localization resources
• add complete Simplified Chinese (zh-CN) resources with English fallback
• detect the Windows UI language and support System, English, and Simplified Chinese preferences
• persist manual language overrides in Settings
• refresh the widget, Settings, tray, taskbar, activity-hook, tooltip, accessibility, status, error, and update UI at runtime
• keep existing numeric and date formatting semantics unchanged
• add resource parity, culture resolution, persistence, runtime switching, and regression tests

No third-party localization dependencies were added.

Testing

• dotnet restore .\CodexUsageWidget.slnx
• dotnet build .\CodexUsageWidget.slnx -c Release --no-restore
• dotnet test .\CodexUsageWidget.slnx -c Release --no-build (157 tests passed)
• manually verified English, Simplified Chinese, runtime switching, persistence, and Windows UI layout

Closes #4

[Jill-Stingray-L]

Oh, by the way, would you be open to a separate follow-up PR adding a Simplified Chinese README as README.zh-CN.md? I’d keep it outside this PR so the current localization changes remain focused.

[ognjeeen]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Apply the selected culture to formatted values

When an en-US user selects Simplified Chinese, or a zh-CN user selects English, this updates only CurrentUICulture; visible values are still formatted with CultureInfo.CurrentCulture in UsageTextFormatter, TokenActivityViewModel, UsageWidgetViewModel, and TaskbarLabelWindow. Consequently, day and month names remain in the Windows regional language (for example, an English Sunday, August 30 inside the Chinese UI), contradicting the explicit “always use” language setting. Also update the formatting culture/default, or format these values using Strings.Current.Culture.

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