fleet-manager: checkpoint 6 — the admin SPA (fleet command deck)

The themed single-page admin UI (serving/fleet-manager/ui/), embedded into the
binary and served same-origin over the admin plane.

- Svelte 5 + Tailwind 4 + Vite; a sibling of serving/kiosk-app. Theme = LYSANDRA
  'restricted-violet' (high-security evolution of the kiosk's flight-deck DNA):
  amber deck, VIOLET reserved for privileged/destructive + auth surfaces, teal
  telemetry, red alert, clearance chrome (corner-reticle panels, scanlines, mono
  clearance labels, secure-link status bar).
- Passkey gate: Login (usernameless discoverable WebAuthn) + Enrol (from the
  register-admin one-time link) via navigator.credentials, with base64url<->buffer
  helpers and the go-webauthn response shapes.
- Three pillars over the JSON API: Monitor (fleet roster + version-drift + health
  + per-host detail with bespoke SVG telemetry charts), Authorise (pending queue:
  fingerprint cross-check, approve+bind in violet, deny), Lifecycle (desired-state
  form; drain/reboot confirm-gated). Session-aware fetch client (cookie + CSRF).
- go:embed behind the  build tag (spa_embed.go / spa_stub.go) so the
  default go build/test need no dist/; mounted at / (api/auth/healthz win).

Verified: npm build clean (71KB JS / 33KB CSS + self-hosted fonts); default +
-tags embedspa go builds; smoke test serves index + assets + API through the SPA
origin. Against an insecure+seeded backend the deck renders passkey-free for
theme work. dist/ + node_modules gitignored.

serving/fleet-manager/main.go

@@ -202,6 +202,9 @@ func main() {
 	apiMux := http.NewServeMux()
 	s.apiRoutes(apiMux)
 	root.Handle("/api/", s.requireSession(apiMux))
+	// SPA at the root (catch-all; the more specific /api, /auth, /healthz win).
+	// Embedded only in `-tags embedspa` builds; a stub otherwise (see spa_*.go).
+	root.Handle("/", s.spaHandler())
 
 	srv := &http.Server{
 		Addr:              cfg.adminAddr,