Fix preview-mode logout when opening account settings (#1670)

## Problem

In **preview mode**, clicking the user profile → **Account settings**
logs the user out and strands them on a sign-in page they can't get
past.

Root cause is a combination of three things:

- Preview mode uses an **in-memory** token store (`tokenStore:
"memory"`), so any full page reload wipes the session.
- The account-settings menu item calls
`app.redirectToAccountSettings()`, whose `redirectTo*` helpers fall
through to `window.location.assign` on the client — a **hard reload**.
- The `/handler/account-settings` route lives **outside** the
`(protected)` layout that performs the preview auto-login, so nothing
re-mints the preview session after the reload.

Net effect: hard nav → in-memory session wiped → `useUser({ or:
'redirect' })` finds no user → redirect to sign-in, with no way back in
(preview credentials are a throwaway UUID).

In normal (cookie) mode this is harmless because the cookie survives the
reload — the bug is preview-specific.

## Fix

Soft-navigate from the menu item using `app.useNavigate()` (a
`router.push` wrapper under the `"nextjs"` redirect method) instead of
`redirectToAccountSettings()`. A soft client-side navigation does not
re-evaluate the JS bundle, so the module-singleton app instance and its
in-memory token store stay alive.

The fix is scoped to the single dashboard call site rather than the
shared SDK `redirectToAccountSettings()` method, which ships to all
customers and intentionally hard-navigates for cross-domain / sign-out
flows.

## Note / follow-up

This covers navigation **from within the app**, which is the only in-app
entry point to account settings. It does **not** cover a hard refresh or
a direct link to `/handler/account-settings` in preview mode — those
still wipe the in-memory session because the handler route has no
auto-login. A fuller root-cause fix would bring the preview auto-login
(or the `(protected)` layout) to the handler route.

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fixes preview-mode logout when opening Account settings by switching the
dashboard menu to a soft client-side nav using `app.useNavigate()`
instead of a hard reload. This preserves the in-memory session.

- **Bug Fixes**
- Soft-navigate to `/handler/account-settings` via `app.useNavigate()`
to avoid wiping the preview token.
- Change scoped to the dashboard menu; `redirectToAccountSettings()` in
the SDK remains unchanged.

<sup>Written for commit 0e0c3a1bab.
Summary will update on new commits.</sup>

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* The “Account settings” menu now opens with smooth in-app navigation
instead of a full page reload, making the experience faster and more
seamless.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/dashboard/src/components/dashboard-user-button.tsx

@@ -88,6 +88,9 @@ export function DashboardUserButton(props: DashboardUserButtonProps) {
 function DashboardUserButtonInner(props: DashboardUserButtonProps) {
   const user = useUser();
   const app = useStackApp();
+  // Soft-navigate: redirectToAccountSettings() hard-reloads, which wipes preview mode's
+  // in-memory session and bounces the user to sign-in. router.push keeps the session alive.
+  const navigate = app.useNavigate();
   const showUserInfo = props.showUserInfo === true;
   const displayName = user?.displayName ?? user?.primaryEmail ?? "Account";
   const iconProps = { size: 16, className: menuIconClassName };
@@ -142,7 +145,7 @@ function DashboardUserButtonInner(props: DashboardUserButtonProps) {
         {user && (
           <DashboardMenuItem
             text="Account settings"
-            onClick={async () => await app.redirectToAccountSettings()}
+            onClick={() => navigate("/handler/account-settings")}
             icon={<UserCircleIcon {...iconProps} />}
           />
         )}