fix sidebar icon rail overflow

The rail was a fixed Row(center, max) — one button per tab — so adding
the Search tab pushed it 54px past its width and threw a RenderFlex
overflow. Center the icons when they fit and scroll horizontally when
they don't (LayoutBuilder + SingleChildScrollView + a minWidth floor),
so the rail stays correct at any tab count. (T-200)

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

test/widgets/zero_coverage_widgets_test.dart

@@ -349,6 +349,28 @@ void main() {
       await tester.tap(find.bySemanticsLabel('B'));
       expect(selected, 'b');
     });
+
+    testWidgets('scrolls instead of overflowing when items exceed the width', (tester) async {
+      await tester.pumpWidget(harness(
+        f,
+        SizedBox(
+          width: 120,
+          height: 30,
+          child: ClideIconRail(
+            items: [
+              for (var i = 0; i < 10; i++) ClideIconRailItem(id: '$i', icon: PhosphorIcons.folder, tooltip: 'Tab $i'),
+            ],
+            activeId: '0',
+            onSelect: (_) {},
+          ),
+        ),
+      ));
+      await tester.pumpAndSettle();
+      // No RenderFlex overflow (would surface as a thrown FlutterError),
+      // and the rail is horizontally scrollable.
+      expect(tester.takeException(), isNull);
+      expect(find.byType(SingleChildScrollView), findsWidgets);
+    });
   });
 
   group('ClideSpine', () {