fix: replace invalid TanStack Router search casts with typed defaults

Newer TanStack Router enforces strict types on search params — 'search: {} as Record<string, unknown>' no longer satisfies routes with validateSearch. Replace all occurrences with the correct search shape for each destination route (pagination defaults for list routes, tab/field defaults for detail routes).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/admin/src/routes/_authenticated/lessons/sessions/$sessionId.tsx

@@ -126,7 +126,7 @@ function SessionDetailPage() {
   return (
     <div className="space-y-6 max-w-3xl">
       <div className="flex items-center gap-3">
-        <Button variant="ghost" size="sm" onClick={() => navigate({ to: '/lessons/sessions', search: {} as Record<string, unknown> })}>
+        <Button variant="ghost" size="sm" onClick={() => navigate({ to: '/lessons/sessions', search: { page: 1, limit: 25, q: undefined, sort: undefined, order: 'asc' as const } })}>
           <ArrowLeft className="h-4 w-4" />
         </Button>
         <div className="flex-1">
@@ -137,7 +137,7 @@ function SessionDetailPage() {
             <Link
               to="/lessons/enrollments/$enrollmentId"
               params={{ enrollmentId: enrollment.id }}
-              search={{} as Record<string, unknown>}
+              search={{ tab: 'details' }}
               className="text-sm text-primary hover:underline"
             >
               View Enrollment