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/kernel/src/reader_nav.dart

@@ -87,6 +87,17 @@ class ReaderNav extends ChangeNotifier {
     notifyListeners();
   }
 
+  /// Toggle the pinned state: pin the current entry when nothing is
+  /// pinned, otherwise clear the pin. (The reader's left-hand pin button.)
+  void togglePin() {
+    if (_pinnedIndex != null) {
+      _pinnedIndex = null;
+      notifyListeners();
+    } else {
+      pin();
+    }
+  }
+
   /// Jump to the pinned entry and re-emit it. No-op when nothing is pinned.
   void jumpToPin() {
     final p = _pinnedIndex;