fix(AppShell): drop t("menu.account") fallback, use email instead

The fallback for users without a displayName was hitting next-intl's
MISSING_MESSAGE error when the consumer didn't ship a `menu.account`
key. Default to the user's email (or empty string) instead of a
translation key the chrome can't guarantee.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/layout/AppShell.tsx

@@ -154,7 +154,7 @@ export function AppShell({
           aria-expanded="false"
         >
           <span className="d-none d-md-inline me-2">
-            {config.user.displayName || t("menu.account")}
+            {config.user.displayName || config.user.email || ""}
           </span>
           <i className="ph-user-circle" />
         </button>