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/spa_stub.go

@@ -0,0 +1,17 @@
+//go:build !embedspa
+
+// spa_stub.go — the default build has no SPA embedded (so `go build`/`go test`
+// don't need ui/dist). Build with `-tags embedspa` after building the SPA to
+// serve the real UI (see spa_embed.go).
+package main
+
+import "net/http"
+
+func (s *server) spaHandler() http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		w.WriteHeader(http.StatusNotFound)
+		_, _ = w.Write([]byte("fleet-manager: SPA not embedded in this build.\n" +
+			"Build with: (cd ui && npm ci && npm run build) then `go build -tags embedspa`.\n"))
+	})
+}