feat: image viewer and prev next nav for MAUI and Avalonia

[Shewart]

Summary

The last two ShellUI Blazor components that can be ported today, each for MAUI and Avalonia. The registry now has 107 entries (104 CLI targets, 73 families).

• image-viewer (ImageViewer): a thumbnail that opens the image over the page on a dark backdrop, with zoom out, reset, zoom in and close controls. Zoom runs from 50% to 400% and resets each time it opens. The backdrop or Escape closes it.
• prev-next-nav (PrevNextNav): two bordered cards for the foot of a page, each a small "Previous" or "Next" over the page's name. A side with no label is hidden.

Both demos show the new components, and the docs cover them, including how the Avalonia versions differ.

Notes

• Zooming scales the image around its centre; it does not pan, like the Blazor version.
• MAUI adds pinch to zoom. Avalonia adds the mouse wheel and the + / - / 0 keys.
• On Avalonia the thumbnail, the viewer controls and the prev/next cards are tab stops.
• Source is an ImageSource on MAUI and an IImage (a resource path) on Avalonia; the thumbnail fit is Aspect on MAUI and Stretch on Avalonia.
• What remains of the Blazor set is the chart family, which needs a charting decision, and qr-code, which needs NuGet dependencies in the CLI.

Verification

• Solution builds with 0 warnings and 0 errors on all four MAUI targets; 1020 tests pass; format is clean.
• Each component installs alone into a fresh Avalonia app, and both together into a fresh dotnet new maui app (Windows target), building with 0 warnings.
• MAUI demo checked on Windows in dark and light.
• Avalonia demo checked on Windows in dark and light with pointer, key and wheel input.