Remove unused SidebarField; add CI typecheck job before client Docker build

- Delete the now-unused SidebarField component from TicketDetail.tsx
- Add typecheck-client CI job that runs tsc --noEmit on the client before
  the Docker build, so TypeScript errors surface fast with a clear message
- build-client now depends on typecheck-client passing

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

client/src/pages/TicketDetail.tsx

@@ -55,15 +55,6 @@ const AUDIT_COLORS: Record<string, string> = {
 const COMMENT_ACTIONS = new Set(['COMMENT_ADDED', 'COMMENT_DELETED'])
 
 
-function SidebarField({ label, children }: { label: string; children: React.ReactNode }) {
-  return (
-    <div>
-      <p className="text-xs font-medium text-gray-500 mb-1.5">{label}</p>
-      {children}
-    </div>
-  )
-}
-
 export default function TicketDetail() {
   const { id } = useParams<{ id: string }>()
   const navigate = useNavigate()