Rename maglev-frontend → maglevd-frontend; v0.9.1; API RX/TX pulse

Rename the web dashboard binary to maglevd-frontend and move it to
/usr/sbin (it's a daemon and belongs with maglevd). The systemd unit
name stays vpp-maglev-frontend.service since that prefix is the
package name. Manpage, README, user-guide, and debian packaging all
updated in lockstep; bump to 0.9.1 for the first real release.

All frontend env vars are now prefixed MAGLEV_FRONTEND_ so a single
/etc/default/vpp-maglev can be shared with maglevd without collisions.
Every flag has an env equivalent for Docker use. MAGLEV_FRONTEND_USER
and MAGLEV_FRONTEND_PASSWORD still gate the /admin surface.

VPPInfoPanel now pulses "API: ↑↓" indicators in the zippy title
whenever a vpp-api-send / vpp-api-recv log event arrives on the SSE
stream for the scoped maglevd — 250ms blue flash, re-triggerable,
with the two arrows tightly kerned via negative letter-spacing.

cmd/frontend/handlers.go

@@ -106,7 +106,7 @@ func registerHandlers(mux *http.ServeMux, clients []*maglevClient, broker *Broke
 		adminAPI := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			handleAdminAPI(w, r, byName)
 		})
-		realm := "maglev-frontend admin"
+		realm := "maglevd-frontend admin"
 		// Register /admin/api/ before /admin/ so the more specific
 		// pattern wins in net/http's ServeMux.
 		mux.Handle("/admin/api/", basicAuth(realm, admin.User, admin.Password, adminAPI))