style: move collapsed sidebar expand control to top

lib/widgets/sidebar_navigation.dart

@@ -106,6 +106,13 @@ class SidebarNavigation extends StatelessWidget {
           crossAxisAlignment: CrossAxisAlignment.stretch,
           children: [
             const SizedBox(height: 4),
+            if (isCollapsed && showCollapseControl) ...[
+              SidebarHeader(
+                isCollapsed: true,
+                onTap: onExpandFromCollapsed,
+              ),
+              const SizedBox(height: AppSpacing.xs),
+            ],
             Expanded(
               child: Column(
                 crossAxisAlignment: CrossAxisAlignment.stretch,
@@ -151,7 +158,7 @@ class SidebarNavigation extends StatelessWidget {
                         ),
                     accountSelected:
                         currentSection == WorkspaceDestination.account,
-                    showCollapseControl: showCollapseControl && isCollapsed,
+                    showCollapseControl: false,
                   ),
                 ],
               ),
@@ -189,6 +196,7 @@ class SidebarHeader extends StatelessWidget {
       child: Align(
         alignment: Alignment.centerRight,
         child: InkWell(
+          key: const Key('sidebar-header-expand-button'),
           borderRadius: BorderRadius.circular(AppRadius.button),
           onTap: onTap,
           child: Padding(padding: EdgeInsets.zero, child: content),

test/widgets/sidebar_navigation_suite.dart

@@ -160,6 +160,47 @@ void main() {
     );
   });
 
+  testWidgets('SidebarNavigation shows collapsed expand button at the top', (
+    WidgetTester tester,
+  ) async {
+    var expanded = 0;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: AppTheme.light(),
+        home: Scaffold(
+          body: SidebarNavigation(
+            currentSection: WorkspaceDestination.assistant,
+            sidebarState: AppSidebarState.collapsed,
+            appLanguage: AppLanguage.zh,
+            themeMode: ThemeMode.light,
+            onSectionChanged: (_) {},
+            onToggleLanguage: () {},
+            onCycleSidebarState: () {},
+            onExpandFromCollapsed: () => expanded++,
+            onOpenHome: () {},
+            onOpenAccount: () {},
+            onOpenThemeToggle: () {},
+            accountName: 'Tester',
+            accountSubtitle: 'Workspace',
+            onToggleAccountWorkspaceFollowed: () async {},
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.byKey(const Key('sidebar-header-expand-button')), findsOneWidget);
+    expect(
+      find.byKey(const ValueKey<String>('sidebar-footer-collapse')),
+      findsNothing,
+    );
+
+    await tester.tap(find.byKey(const Key('sidebar-header-expand-button')));
+    await tester.pumpAndSettle();
+    expect(expanded, 1);
+  });
+
   testWidgets('SidebarNavigation merges task controls into the global left bar', (
     WidgetTester tester,
   ) async {