refactor(ui): one canonical segment control (ChainSegments) everywhere

The same single-select choice pattern appeared as four widgets:
audit's hover pills, the store's SegmentedButton, the store filter
dialog's ChoiceChips, and the approvals TabBar (usertest finding
#14 / night-log decision 'pill segment as canon'). The audit
pattern is promoted to a shared ChainSegments widget (optional
icons, hover, selected border, button+selected semantics) and all
four sites use it; approvals switches lists via IndexedStack so
both stay alive and switching does not refetch.

Guard per the no-bugfix-without-a-guard rule: widget tests for
selection + semantics, plus a canon sweep that bans
TabBar/TabBarView/TabController/SegmentedButton/ChoiceChip from
lib/ (comments exempt). Deliberately out of scope: the flow
editor's Graph/Text/Run tabs live in the separate editor package.
Studio 0.75.0; guide images regenerated, dark + light verified.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

test/chain_segments_test.dart

@@ -0,0 +1,101 @@
+// ChainSegments — the canonical single-select segment control
+// (night-log decision "pill segment as canon"; usertest finding
+// #14: the same choice pattern appeared as four different
+// widgets). Guards both the widget behaviour and the canon
+// itself: no page may reintroduce TabBar / SegmentedButton /
+// ChoiceChip for single-select segments.
+
+import 'dart:io';
+
+import 'package:flutter/material.dart';
+import 'dart:ui' show Tristate;
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:chain_studio/widgets/chain_segments.dart';
+
+Widget _host(Widget child) =>
+    MaterialApp(home: Scaffold(body: Center(child: child)));
+
+void main() {
+  testWidgets('tapping a segment reports the new value once', (tester) async {
+    final changes = <String>[];
+    await tester.pumpWidget(
+      _host(
+        ChainSegments<String>(
+          items: const [
+            ChainSegmentItem('all', 'Alle'),
+            ChainSegmentItem('flow', 'Flow'),
+            ChainSegmentItem('step', 'Schritt'),
+          ],
+          value: 'all',
+          onChanged: changes.add,
+        ),
+      ),
+    );
+    await tester.tap(find.text('Flow'));
+    await tester.pump();
+    expect(changes, ['flow']);
+    // Tapping the already-selected value must NOT re-fire — call
+    // sites use the callback to trigger refetches.
+    await tester.tap(find.text('Alle'));
+    await tester.pump();
+    expect(changes, ['flow']);
+  });
+
+  testWidgets('segments expose button + selected semantics', (tester) async {
+    final handle = tester.ensureSemantics();
+    await tester.pumpWidget(
+      _host(
+        ChainSegments<int>(
+          items: const [
+            ChainSegmentItem(0, 'Ausstehend'),
+            ChainSegmentItem(1, 'Verlauf'),
+          ],
+          value: 0,
+          onChanged: (_) {},
+        ),
+      ),
+    );
+    final pending = tester.getSemantics(find.text('Ausstehend'));
+    expect(pending.flagsCollection.isSelected, Tristate.isTrue);
+    final history = tester.getSemantics(find.text('Verlauf'));
+    expect(history.flagsCollection.isSelected, Tristate.isFalse);
+    handle.dispose();
+  });
+
+  test('canon guard: no page reintroduces TabBar/SegmentedButton/ChoiceChip',
+      () {
+    // The night-log decision made the pill segment the ONE way to
+    // render a single-select segment. This sweep keeps the three
+    // retired Material patterns out of lib/ for good (comments
+    // don't count; the widget file itself is exempt).
+    final offenders = <String>[];
+    final banned = RegExp(
+      r'\b(TabBar|TabBarView|TabController|SegmentedButton|ChoiceChip)\b',
+    );
+    final files = Directory('lib')
+        .listSync(recursive: true)
+        .whereType<File>()
+        .where((f) => f.path.endsWith('.dart'))
+        .where((f) => !f.path.contains('chain_segments.dart'));
+    for (final f in files) {
+      final lines = f.readAsLinesSync();
+      for (var i = 0; i < lines.length; i++) {
+        final line = lines[i];
+        final code = line.contains('//')
+            ? line.substring(0, line.indexOf('//'))
+            : line;
+        if (banned.hasMatch(code)) {
+          offenders.add('${f.path}:${i + 1}: ${line.trim()}');
+        }
+      }
+    }
+    expect(
+      offenders,
+      isEmpty,
+      reason:
+          'Single-select segments use ChainSegments (canon). '
+          'Found retired patterns:\n${offenders.join('\n')}',
+    );
+  });
+}