Enable text selection in HTML message content

[Copilot]

Text in posts, comments, conversations, and profile bios was not selectable — long-press had no effect. This was previously blocked by flutter/flutter#43494 (missing SelectableRegion support), which has since landed.

Change

• Wrap the Html widget in Message with Flutter's SelectionArea, making all rendered text selectable via long-press with no changes to link tap behaviour

// Before
return Html(shrinkWrap: true, ...);

// After
return SelectionArea(child: Html(shrinkWrap: true, ...));

Applies everywhere Message is used: stream, post detail, comments, conversations, and profile pages.