Fix log text color for dark mode and live theme switch

[Gabriel Dufresne (GabrielDuf)]

This pull request improves how the application handles theming for log and output views, ensuring that colors and brushes adapt correctly to theme changes (such as switching between light and dark mode) at runtime. The changes introduce a new ThemeHelper utility, refactor brush resolution logic, and update UI components to recolor output lines dynamically when the theme changes.

The most important changes are:

Theme detection and brush resolution improvements:

• Added a new static class ThemeHelper in src/UniGetUI.Avalonia/Infrastructure/ThemeHelper.cs to reliably determine the current theme variant and whether the app is in dark mode, resolving issues where theme detection could be inconsistent.
• Refactored all theme and brush lookups in log/output-related view models and dialogs to use ThemeHelper for consistent theme resolution, replacing direct use of Application.Current.ActualThemeVariant.

Dynamic theme switching support:

• Updated OperationOutputViewModel, OperationFailedDialog, OperationOutputWindow, and BaseLogPage to listen for theme changes (ActualThemeVariantChanged) and rebuild or reload their output lines with the correct brushes, so already-rendered output adapts instantly to theme switches.

Code cleanup and reliability:

• Removed redundant or direct resource lookups in favor of the new centralized brush lookup logic, and cleaned up unused imports related to Avalonia theming.

These changes make the application's log and output views more visually reliable and responsive to theme changes, addressing previous issues with incorrect colors in dark mode.