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/src/components/Panel.svelte

@@ -0,0 +1,13 @@
+<script>
+  let { title = '', tag = '', restricted = false, children } = $props()
+</script>
+
+<section class="panel {restricted ? 'panel-restricted' : ''} p-4">
+  {#if title || tag}
+    <header class="mb-3 flex items-baseline justify-between gap-3">
+      <h2 class="font-display text-sm {restricted ? 'text-violet' : 'text-amber'}">{title}</h2>
+      {#if tag}<span class="tag">{tag}</span>{/if}
+    </header>
+  {/if}
+  {@render children?.()}
+</section>