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/ui/README.md

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+# fleet-manager UI — the fleet command deck
+
+The admin SPA for `fleet-manager`. **Svelte 5 + Tailwind 4 + Vite**, themed
+**LYSANDRA / restricted-violet** (a high-security evolution of the kiosk-app's
+LYSANDRA flight-deck: amber deck, **violet reserved for privileged/destructive +
+auth surfaces**, teal telemetry, red alert, clearance chrome). A sibling of
+`serving/kiosk-app`, kept in-repo for now (may graduate to its own submodule).
+
+Three pillars over the manager's JSON API, plus the passkey gate:
+
+- **Login / Enrol** — WebAuthn ceremonies (`navigator.credentials`). Enrol is
+  reached via the one-time link from `fleet-manager register-admin`.
+- **Monitor** — fleet roster (online / version-drift / health) + per-host detail
+  with bespoke SVG telemetry charts (from `/api/hosts/{serial}/metrics`).
+- **Authorise** — pending-enrolment queue: cross-check the fingerprint, approve
+  (violet) + bind, or deny.
+- **Lifecycle** — per-host desired-state (version pin, kiosk URL, reboot, drain);
+  destructive changes are confirm-gated.
+
+## Build / embed
+
+The built SPA is embedded into the `fleet-manager` binary via `go:embed` and
+served same-origin over the admin plane.
+
+```bash
+npm ci
+npm run build          # -> dist/ (gitignored)
+cd .. && go build -tags embedspa   # embeds dist/ into the binary
+```
+
+The default `go build` (no tag) ships a stub instead, so Go builds/tests don't
+require `dist/`. The Dockerfile does the two-stage build (npm → dist → go build
+`-tags embedspa`).
+
+## Dev
+
+```bash
+npm run dev            # Vite on :5173, proxies /api + /auth to :9500
+```
+
+Point it at a locally-running manager. Against an **insecure+seeded** manager
+(`FLEET_MGR_ADMIN_INSECURE=true FLEET_MGR_SEED=true`), `/api/whoami` returns
+`dev-insecure`, so the SPA renders the pillars **without a passkey** — handy for
+theme/layout work. Passkey login/enrol need the manager in secure mode with a
+valid RP origin + a WebAuthn authenticator.