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/types.go

@@ -25,7 +25,7 @@ type MaglevdInfo struct {
 	LastError string `json:"last_error,omitempty"`
 }
 
-// VersionInfo is the build metadata of this maglev-frontend binary
+// VersionInfo is the build metadata of this maglevd-frontend binary
 // plus runtime capability flags the SPA needs to know at mount time.
 type VersionInfo struct {
 	Version      string `json:"version"`