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>

lib/pages/approvals.dart

@@ -36,9 +36,12 @@ class ApprovalsPage extends StatefulWidget {
   State<ApprovalsPage> createState() => _ApprovalsPageState();
 }
 
-class _ApprovalsPageState extends State<ApprovalsPage>
-    with SingleTickerProviderStateMixin {
-  late final TabController _tab;
+class _ApprovalsPageState extends State<ApprovalsPage> {
+  /// 0 = pending inbox, 1 = history. Plain index instead of a
+  /// TabController: the page uses the canonical pill segment
+  /// (ChainSegments), and an IndexedStack keeps both lists alive
+  /// so switching does not refetch.
+  int _tabIndex = 0;
   late Future<List<ApprovalRecord>> _pendingFuture;
   late Future<List<ApprovalRecord>> _historyFuture;
 
@@ -63,7 +66,6 @@ class _ApprovalsPageState extends State<ApprovalsPage>
   @override
   void initState() {
     super.initState();
-    _tab = TabController(length: 2, vsync: this);
     Workspace.instance.addListener(_refresh);
     Workspace.instance.ensureLoaded();
     _refresh();
@@ -72,7 +74,6 @@ class _ApprovalsPageState extends State<ApprovalsPage>
   @override
   void dispose() {
     Workspace.instance.removeListener(_refresh);
-    _tab.dispose();
     super.dispose();
   }
 
@@ -262,12 +263,33 @@ class _ApprovalsPageState extends State<ApprovalsPage>
       backgroundColor: theme.scaffoldBackgroundColor,
       appBar: AppBar(
         title: Text(AppLocalizations.of(context)!.approvalsTitle),
-        bottom: TabBar(
-          controller: _tab,
-          tabs: [
-            Tab(text: AppLocalizations.of(context)!.approvalsTabPending),
-            Tab(text: AppLocalizations.of(context)!.approvalsTabHistory),
-          ],
+        bottom: PreferredSize(
+          preferredSize: const Size.fromHeight(44),
+          child: Align(
+            alignment: Alignment.centerLeft,
+            child: Padding(
+              padding: const EdgeInsets.fromLTRB(
+                ChainSpace.xl,
+                0,
+                ChainSpace.xl,
+                ChainSpace.sm,
+              ),
+              child: ChainSegments<int>(
+                items: [
+                  ChainSegmentItem(
+                    0,
+                    AppLocalizations.of(context)!.approvalsTabPending,
+                  ),
+                  ChainSegmentItem(
+                    1,
+                    AppLocalizations.of(context)!.approvalsTabHistory,
+                  ),
+                ],
+                value: _tabIndex,
+                onChanged: (i) => setState(() => _tabIndex = i),
+              ),
+            ),
+          ),
         ),
         actions: [
           const ChainWorkspaceSwitcher(),
@@ -285,8 +307,8 @@ class _ApprovalsPageState extends State<ApprovalsPage>
           const SizedBox(width: ChainSpace.sm),
         ],
       ),
-      body: TabBarView(
-        controller: _tab,
+      body: IndexedStack(
+        index: _tabIndex,
         children: [
           _PendingList(
             future: _pendingFuture,