feat: impersonation system, semi-finalist detail page, tRPC resilience

- Add super-admin impersonation: "Login As" from user list, red banner
  with "Return to Admin", audit logged start/end, nested impersonation
  blocked, onboarding gate skipped during impersonation
- Fix semi-finalist stats: check latest terminal state (not any PASSED),
  use passwordHash OR status=ACTIVE for activation check
- Add /admin/semi-finalists detail page with search, category/status filters
- Add account_reminder_days setting to notifications tab
- Add tRPC resilience: retry on 503/HTML responses, custom fetch detects
  nginx error pages, exponential backoff (2s/4s/8s)
- Reduce dashboard polling intervals (60s stats, 30s activity, 120s semi)

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

src/components/shared/impersonation-banner.tsx

@@ -0,0 +1,51 @@
+'use client'
+
+import { useSession } from 'next-auth/react'
+import { useRouter } from 'next/navigation'
+import { trpc } from '@/lib/trpc/client'
+import { Button } from '@/components/ui/button'
+import { ArrowLeft, Loader2 } from 'lucide-react'
+import { toast } from 'sonner'
+
+export function ImpersonationBanner() {
+  const { data: session, update } = useSession()
+  const router = useRouter()
+  const endImpersonation = trpc.user.endImpersonation.useMutation()
+
+  if (!session?.user?.impersonating) return null
+
+  const handleReturn = async () => {
+    try {
+      await endImpersonation.mutateAsync()
+      await update({ endImpersonation: true })
+      toast.success('Returned to admin account')
+      router.push('/admin/members')
+      router.refresh()
+    } catch (error) {
+      toast.error(error instanceof Error ? error.message : 'Failed to end impersonation')
+    }
+  }
+
+  return (
+    <div className="fixed top-0 left-0 right-0 z-50 flex items-center justify-center gap-3 bg-red-600 px-4 py-1.5 text-sm text-white shadow-md">
+      <span>
+        Impersonating <strong>{session.user.name || session.user.email}</strong>{' '}
+        ({session.user.role.replace('_', ' ')})
+      </span>
+      <Button
+        size="sm"
+        variant="secondary"
+        className="h-6 px-3 text-xs"
+        onClick={handleReturn}
+        disabled={endImpersonation.isPending}
+      >
+        {endImpersonation.isPending ? (
+          <Loader2 className="mr-1 h-3 w-3 animate-spin" />
+        ) : (
+          <ArrowLeft className="mr-1 h-3 w-3" />
+        )}
+        Return to Admin
+      </Button>
+    </div>
+  )
+}