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>

test/kernel/reader_nav_test.dart

@@ -115,6 +115,15 @@ void main() {
     expect(loads.last, 'D-1');
   });
 
+  test('togglePin pins the current entry, then clears it', () {
+    nav.open('D-1');
+    expect(nav.hasPinned, isFalse);
+    nav.togglePin();
+    expect(nav.hasPinned, isTrue);
+    nav.togglePin();
+    expect(nav.hasPinned, isFalse);
+  });
+
   test('pin replaces the previous pin', () {
     nav.open('D-1');
     nav.pin();