fix(dashboard): Remove 308 redirect when creating new dashboards

Frontend dashboard-creation entry points navigate to /dashboard/new
while the backend exposes /dashboard/new/, so Flask responds with a 308
permanent redirect to the trailing-slash version. In some infrastructure
setups that redirect resolves to an invalid address and breaks the
create-dashboard flow.

Point all create-dashboard links/buttons (welcome empty-state, right-hand
menu, dashboard table, dashboard list) directly at /dashboard/new/ so the
browser hits the backend endpoint without a 308. Tests updated to match.

Adopts #28754, original by @ericsong. Related to #20577.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

superset-frontend/src/features/home/DashboardTable.test.tsx

@@ -243,7 +243,7 @@ test('handles create dashboard button click', async () => {
 
   const createButton = screen.getByRole('button', { name: /dashboard$/i });
   await userEvent.click(createButton);
-  expect(assignMock).toHaveBeenCalledWith('/dashboard/new');
+  expect(assignMock).toHaveBeenCalledWith('/dashboard/new/');
   locationSpy.mockRestore();
 });
 

superset-frontend/src/features/home/DashboardTable.tsx

@@ -203,7 +203,7 @@ function DashboardTable({
             name: t('Dashboard'),
             buttonStyle: 'secondary',
             onClick: () => {
-              navigateTo('/dashboard/new', { assign: true });
+              navigateTo('/dashboard/new/', { assign: true });
             },
           },
           {

superset-frontend/src/features/home/EmptyState.tsx

@@ -57,7 +57,7 @@ const LABELS = {
 const REDIRECTS = {
   create: {
     [WelcomeTable.Charts]: '/chart/add',
-    [WelcomeTable.Dashboards]: '/dashboard/new',
+    [WelcomeTable.Dashboards]: '/dashboard/new/',
     // navigateTo() applies the application root internally; keep this
     // relative so the prefix isn't added twice.
     [WelcomeTable.SavedQueries]: '/sqllab?new=true',

superset-frontend/src/features/home/Menu.test.tsx

@@ -89,7 +89,7 @@ const dropdownItems = [
   },
   {
     label: 'Dashboard',
-    url: '/dashboard/new',
+    url: '/dashboard/new/',
     icon: 'fa-fw fa-dashboard',
     perm: 'can_write',
     view: 'Dashboard',

superset-frontend/src/features/home/RightMenu.test.tsx

@@ -105,7 +105,7 @@ const dropdownItems = [
   },
   {
     label: 'Dashboard',
-    url: '/dashboard/new',
+    url: '/dashboard/new/',
     icon: 'fa-fw fa-dashboard',
     perm: 'can_write',
     view: 'Dashboard',

superset-frontend/src/features/home/RightMenu.tsx

@@ -232,7 +232,7 @@ const RightMenu = ({
     },
     {
       label: t('Dashboard'),
-      url: '/dashboard/new',
+      url: '/dashboard/new/',
       icon: (
         <Icons.DashboardOutlined data-test={`menu-item-${t('Dashboard')}`} />
       ),

superset-frontend/src/pages/DashboardList/index.tsx

@@ -762,7 +762,7 @@ function DashboardList(props: DashboardListProps) {
       name: t('Dashboard'),
       buttonStyle: 'primary',
       onClick: () => {
-        navigateTo('/dashboard/new', { assign: true });
+        navigateTo('/dashboard/new/', { assign: true });
       },
     });
   }