Localize literal strings

[Copilot]

Two hardcoded Chinese strings in MainWindow.xaml — the GitHub button label and the messages count suffix — were not yet localized. Also switches TextResource code generation to public so the resource class can be referenced directly from XAML via {x:Static}.

Changes

• Mo3RegUI.csproj: ResXFileCodeGenerator → PublicResXFileCodeGenerator to expose TextResource as a public class
• TextResource.resx / TextResource.zh-Hans.resx: Added two new keys:
  • MainWindow_GitHubUrlButton — "View on GitHub" / "在 GitHub 上查看"
  • MainWindow_MessagesCount — " messages" / " 条消息"
• MainWindow.xaml: Added xmlns:l10n namespace alias; replaced both Chinese literals with {x:Static} bindings:

xmlns:l10n="clr-namespace:Mo3RegUI.LocalizationResources"

<!-- GitHub button -->
<Label Margin="0" Content="{x:Static l10n:TextResource.MainWindow_GitHubUrlButton}"/>

<!-- Messages count suffix -->
<TextBlock Text="{x:Static l10n:TextResource.MainWindow_MessagesCount}" ... />

TextResource.Designer.cs regeneration (to pick up the public modifier and the two new properties) is left to the repository owner.

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