fix: use vscode theme variables for error panel colors

[marlonwq]

This PR fixes an issue where the error panel in notebook cells hardcoded its background and text colors, making it unreadable on custom VS Code themes like Catppuccin.

I added CSS fallback chaining to the .verso-output--error class. It now prioritizes the native --vscode-inputValidation-errorBackground and --vscode-inputValidation-errorForeground tokens, falling back to the Verso tokens and static hex colors if the VS Code variables are missing (e.g., when running in the browser).

Fixes catppuccin/vscode#640

[TorreyBetts]

Thanks for submitting a PR to fix this issue. The fix belongs in a different spot, though.

In VS Code, the --verso-* variables are populated by the mapping block in vscode/src/blazor/blazorEditorProvider.ts (the :root style in the webview template). The bug is that it pairs inputValidation.errorBackground with errorForeground. On Catppuccin both are reds, making it unreadable.

Could you:

1. Drop both app.css changes (the hex values there are fallbacks only and never apply in VS Code, and the browser host has no --vscode-* variables).
2. Change the foreground line in blazorEditorProvider.ts to:

--verso-cell-error-foreground: var(--vscode-inputValidation-errorForeground, var(--vscode-errorForeground, #F48771));

This keeps the change scoped to VS Code, and doesn't mingle with Blazor server that's separate from the VS Code extension.

[marlonwq]

Thanks for pointing me in the right direction! I updated the PR with your suggestions.

[TorreyBetts]

Thanks for your contribution. This fix will be released in 1.0.22 or 1.1.0. We're juggling between the two because some important changes are nearly done.