admin: also register issue-desktop on RegisterOnMux

The Stockyard binary deploys two HTTP server entrypoints in one
process: standalone apiserver via s.mux (used by tests + isolated
runs) and the unified-binary mount via RegisterOnMux (used in
production where billing/api share a port with the marketing site
and the 6 flagship apps).

The first commit of this endpoint (09298d1) only registered the
new route in s.mux. Production Railway deploy serves via
RegisterOnMux, so the route 404'd in production while passing in
local-mux probes.

Symptom that surfaced this: POST /api/admin/issue-desktop with no
auth returned the proper 401 (s.mux somehow served), but with
auth header it 404'd with the marketing site's not-found page —
that 401 was actually from a different code path; the 404 came
from RegisterOnMux not knowing the route. Subtle.

Fix: register on the RegisterOnMux block too, matching what the
existing /api/admin/issue does (it has identical dual registration).

Author: Claude

internal/apiserver/server.go

@@ -307,6 +307,7 @@ func (s *Server) RegisterOnMux(mux *http.ServeMux) {
 	mux.HandleFunc("GET /api/admin/stats", s.adminAuth(s.handleAdminStats))
 	mux.HandleFunc("GET /api/admin/licenses", s.adminAuth(s.handleAdminLicenses))
 	mux.HandleFunc("POST /api/admin/issue", s.adminAuth(s.handleAdminIssue))
+	mux.HandleFunc("POST /api/admin/issue-desktop", s.adminAuth(s.handleAdminIssueDesktop))
 	mux.HandleFunc("POST /api/admin/revoke", s.adminAuth(s.handleAdminRevoke))
 	mux.HandleFunc("POST /api/admin/backup", s.adminAuth(s.handleAdminBackup))
 	mux.HandleFunc("GET /api/admin/trial-drip/suspects", s.adminAuth(s.handleAdminTrialDripSuspects))