Stage 1: project setup, Exercise Library, both Mode designs and the Mode switch

- Flutter Android app on CookIT's stack (provider, shared_preferences, gen-l10n)
- tools/exercise-import turns free-exercise-db into 876 bundled Exercises
  with 1,746 WebP images (20 MB); stretching -> Stretching, rest -> Workout
- ZenStyle tokens per Mode: calm rounded sage/teal with breath ripples vs.
  dark ember-to-red with floor-tape slashes; the theme morphs at each
  Mode's own pace
- Remembered global Mode switch, Plans / Library / History tabs

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

lib/src/design/page_header.dart

@@ -0,0 +1,69 @@
+import 'package:flutter/material.dart';
+
+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});
+
+  final String title;
+  final String? subtitle;
+
+  @override
+  Widget build(BuildContext context) {
+    final s = ZenStyle.of(context);
+    final text = Theme.of(context).textTheme;
+    return Padding(
+      padding: const EdgeInsets.fromLTRB(20, 28, 20, 16),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          Text(title, style: text.headlineLarge),
+          if (subtitle != null) ...[
+            const SizedBox(height: 6),
+            Text(subtitle!, style: text.bodyLarge?.copyWith(color: s.inkMuted)),
+          ],
+        ],
+      ),
+    );
+  }
+}
+
+/// What an empty tab shows: what will appear here and how to get it there.
+class EmptyState extends StatelessWidget {
+  const EmptyState({super.key, required this.icon, required this.title, required this.body});
+
+  final IconData icon;
+  final String title;
+  final String body;
+
+  @override
+  Widget build(BuildContext context) {
+    final s = ZenStyle.of(context);
+    final text = Theme.of(context).textTheme;
+    return Padding(
+      padding: const EdgeInsets.symmetric(horizontal: 20),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          Container(
+            width: 56,
+            height: 56,
+            decoration: BoxDecoration(
+              color: s.surfaceSoft,
+              borderRadius: BorderRadius.circular(s.controlRadius.clamp(0, 28)),
+            ),
+            child: Icon(icon, color: s.accent),
+          ),
+          const SizedBox(height: 16),
+          Text(title, style: text.titleLarge),
+          const SizedBox(height: 6),
+          ConstrainedBox(
+            constraints: const BoxConstraints(maxWidth: 340),
+            child: Text(body, style: text.bodyMedium?.copyWith(color: s.inkMuted)),
+          ),
+        ],
+      ),
+    );
+  }
+}