[FEAT] - Add in-app explanations for comics from ExplainXKCD

[dylanlangston]

Is your feature request related to a problem? Please describe.

Many XKCD comics contain subtle references or jokes that aren't immediately obvious. Users currently have to leave the app and visit explainxkcd.com separately to understand these comics, disrupting the reading experience.

Describe the solution you'd like

Add an in-app feature that displays explanations from ExplainXKCD for each comic. The project already includes a partially implemented client (https://github.com/dylanlangston/Axkcd/blob/master/src/XKCDCore/ExplainXKCDClient.cs) which should be used/extended:

• Complete and enhance ExplainXKCDClient.cs to handle complex wikitext elements (tables, links, basic formatting)
• Parse and sanitize HTML or markdown returned from ExplainXKCD for proper display
• Design and implement a responsive UI (popup/modal/flyout panel) to display explanations within the Avalonia application
• Integrate explanation retrieval into the main comic viewing ViewModel
• Decide whether explanations appear automatically or via button (e.g., "Explain This")
• Cache fetched explanations to reduce repeated API calls and improve performance
• Ensure accessibility and responsive layout for different form factors
• Document the design, usage, and any API limitations

Describe alternatives you've considered

• Opening browser to ExplainXKCD, but this disrupts the in-app experience.

Additional context

Acceptance Criteria:

• Users can view ExplainXKCD explanations within the app
• Explanation UI integrates cleanly into main comic viewer
• Content loads quickly and displays in readable format
• Wikitext is properly parsed and formatted
• Explanations are cached to minimize API calls
• Behavior is consistent across all supported platforms
• Loading states and error handling are implemented

The existing ExplainXKCDClient.cs provides a starting point, making this more approachable for contributors.