ui(settings): use semantic status colors for extension installation errors and notices #1651

Problem

In lib/features/settings/preferences_extensions_section.dart (lines 127–135):

if (_error != null) ...[
  const material.SizedBox(height: 10),
  Text(_error!).small().foreground(),
],
if (_success != null) ...[
  const material.SizedBox(height: 10),
  Text(_success!).muted().small(),
],

• When sideloading an extension fails (e.g. invalid zip, corrupt manifest), _error is styled with standard .foreground() color. It looks like a normal label rather than an error notification.
• Successful installation _success is styled with .muted().small(), appearing disabled/greyed out instead of positive confirmation.

Solution

• Use context.workbench.destructive (or Theme.of(context).colorScheme.destructive) for _error.
• Use context.workbench.success for _success, or display it in a status banner.