Stage 3: Plan editor with Steps, Supersets and reordering

- Plan model with immutable edits: add with Mode defaults, move, remove,
  Superset with next, take a member out, ungroup; Stretching length
  estimate from holds, sides, rounds and Transitions
- drift database (plans table, items as JSON, ADR 0005) and
  PlanRepository; Plans list per Mode with rename/duplicate/delete
- Editor: drag handles, step editor sheet (sets, reps or time, optional
  kg; hold, both sides, rounds; Superset sets and rest between),
  pause between holds for Stretching Plans
- Exercise picker reusing the Library browser; Add to plan from the
  Exercise detail page
- Theme: Mode-styled snackbar, FAB, menus, segmented buttons, switches

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/src/design/page_header.dart

@@ -4,11 +4,14 @@ import 'zen_style.dart';
 
 /// A tab's big title and an optional line under it.
 class PageHeader extends StatelessWidget {
-  const PageHeader({super.key, required this.title, this.subtitle});
+  const PageHeader({super.key, required this.title, this.subtitle, this.action});
 
   final String title;
   final String? subtitle;
 
+  /// A button beside the title, such as "New plan".
+  final Widget? action;
+
   @override
   Widget build(BuildContext context) {
     final s = ZenStyle.of(context);
@@ -18,7 +21,12 @@ class PageHeader extends StatelessWidget {
       child: Column(
         crossAxisAlignment: CrossAxisAlignment.start,
         children: [
-          Text(title, style: text.headlineLarge),
+          Row(
+            children: [
+              Expanded(child: Text(title, style: text.headlineLarge)),
+              if (action != null) ...[const SizedBox(width: 12), action!],
+            ],
+          ),
           if (subtitle != null) ...[
             const SizedBox(height: 6),
             Text(subtitle!, style: text.bodyLarge?.copyWith(color: s.inkMuted)),