move the pin toggle to the pane leading slot

The pin/unpin toggle is a mode control, not navigation — grouping it
with back/forward/jump implied they work alike. Pull it out of
ReaderActionBar into a standalone ReaderPinButton placed before the
title (ClidePaneChrome.leading), leaving the right-hand navigator to
back/forward/jump-to-pin/edit. Applies to all three readers. (T-198)

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

lib/builtin/tickets/src/ticket_detail_view.dart

@@ -73,6 +73,10 @@ class _TicketDetailViewState extends State<TicketDetailView> {
         return ClidePaneChrome(
           title: d.id,
           subtitle: d.title,
+          leading: ReaderPinButton(
+            pinned: _nav?.hasPinned ?? false,
+            onTap: _onPin,
+          ),
           trailing: [
             ReaderActionBar(
               canGoBack: _nav?.canGoBack ?? false,
@@ -80,7 +84,6 @@ class _TicketDetailViewState extends State<TicketDetailView> {
               hasPinned: _nav?.hasPinned ?? false,
               onBack: (_nav?.canGoBack ?? false) ? _onBack : null,
               onForward: (_nav?.canGoForward ?? false) ? _onForward : null,
-              onPin: _onPin,
               onJumpToPin: (_nav?.hasPinned ?? false) ? _onJumpToPin : null,
               onEdit: null, // tickets are pql records, not files
             ),