add status-bar collapse toggles for sidebar + context pane (T-294)

Fixed ~24px caret-line cells bookend the status bar (left=sidebar, right=context);
chevron flips per arrangement.isCollapsed (inward=collapse, outward=expand) and
fires the existing sidebar.collapse / context.collapse commands — the mouse
affordance for the already keyboard/CLI-addressable action (D-6). Visual only;
no new collapse logic. Status items shift 28px inward for the reserved cells.

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

test/app_statusbar_test.dart

@@ -69,9 +69,11 @@ void main() {
     for (final width in [600.0, 3440.0]) {
       await pumpAt(tester, width);
       expect(tester.takeException(), isNull, reason: 'width=$width');
-      // StatusbarHost pads 8px each side → right item's right edge ≈ width - 8.
-      expect(tester.getTopRight(find.text('RIGHT')).dx, closeTo(width - 8, 1.0), reason: 'right group not at edge at width=$width');
-      expect(tester.getTopLeft(find.text('LEFT')).dx, closeTo(8, 1.0), reason: 'left not at start at width=$width');
+      // StatusbarHost pads 4px each side and reserves a fixed 24px collapse-
+      // toggle cell at each end (T-294), so status items sit 28px inside both
+      // edges: right item's right edge ≈ width - 28, left item's left ≈ 28.
+      expect(tester.getTopRight(find.text('RIGHT')).dx, closeTo(width - 28, 1.0), reason: 'right group not at edge at width=$width');
+      expect(tester.getTopLeft(find.text('LEFT')).dx, closeTo(28, 1.0), reason: 'left not at start at width=$width');
     }
   });
 
@@ -83,6 +85,7 @@ void main() {
     ));
     await pumpAt(tester, 3440.0);
     expect(tester.takeException(), isNull);
-    expect(tester.getTopRight(find.text('R2')).dx, closeTo(3440 - 8, 1.0));
+    // 4px pad + 24px reserved toggle cell at the right end (T-294).
+    expect(tester.getTopRight(find.text('R2')).dx, closeTo(3440 - 28, 1.0));
   });
 }