Add Windows dark-mode theming to downloader UI

[Copilot]

High Windows versions should render the downloader in dark mode (black background/white text, matching progress bar) when the OS is in dark theme.

• Dark-mode detection

  • Read AppsUseLightTheme from HKCU\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize; treat value 0 as dark mode.
  • Gracefully fall back to light mode on missing key or access issues (debug-logged in DEBUG builds).

• Theming application

  • Centralized dark brushes; apply to window background/foreground and progress bar background when dark mode is detected.

Example:

private const int DarkModeRegistryValue = 0; // dark mode
if (appsUseLightTheme is int value && value == DarkModeRegistryValue)
{
    Background = DarkBackgroundBrush;
    Foreground = DarkForegroundBrush;
    DownloadProgressBar.Background = DarkBackgroundBrush;
}

Original prompt

尝试适配Windows高版本的深色模式，如果是深色模式则黑底白字，进度条也换成黑底

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[Copilot]

Pull request overview

This PR adds Windows high-version dark mode support to the WPF downloader application. When Windows is in dark mode, the application now detects this via registry settings and applies dark theme colors (black background, white text, and dark progress bar).

• Implements registry-based Windows dark mode detection
• Applies dark theme styling to window and progress bar controls
• Adds comprehensive exception handling for registry access failures

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[Copilot]

The static brush fields defined here are never used in the code. The ApplySystemTheme method directly uses Brushes.Black and Brushes.White instead of these DarkBackgroundBrush and DarkForegroundBrush fields. Either use these fields in ApplySystemTheme or remove them to avoid unnecessary code.

[Copilot]

Setting only the Background property of the ProgressBar to black will not provide proper dark mode styling. In dark mode, the ProgressBar's foreground (the filled portion) should remain visible against the black background. Consider setting the Foreground property of DownloadProgressBar to a light color or adjusting other styling properties to ensure the progress indicator remains visible in dark mode.

[Copilot]

Setting only the window's Background and Foreground properties won't automatically apply dark mode styling to all child controls. The Hyperlink, TextBlocks, and RadioButtons will retain their default styling and may not be readable in dark mode. Consider applying dark theme colors to these controls as well, or use a comprehensive WPF theming solution.