reader pin UX: push-pin icon, toggle, left/right split

Per user feedback on the reader action bar: use the push-pin glyph (not
the chain/link), make the pin button toggle the pinned state (tap to pin
current, tap again to unpin) via ReaderNav.togglePin, and split the
layout so the pin/unpin toggle sits on the left while jump-to-pin joins
the navigator (back/forward) on the right — left toggles, right
navigates. The action button gains an active (accent) state for the
pinned indicator.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/markdown/src/markdown_viewer.dart

@@ -67,7 +67,7 @@ class _MarkdownViewerState extends State<MarkdownViewer> {
 
   void _onBack() => _nav?.back();
   void _onForward() => _nav?.forward();
-  void _onPin() => _nav?.pin();
+  void _onPin() => _nav?.togglePin();
   void _onJumpToPin() => _nav?.jumpToPin();
 
   void _onEdit() {