feat(dashboard): scaffold SvelteKit SPA for control plane

SvelteKit 2 + Svelte 5 (runes) + TS, built with `adapter-static`
into a single SPA bundle that Caddy serves verbatim in production.
The dashboard targets only `/api/admin/*` (manager); data-plane
invocations go through the orchestrator, not through here.

  * Vite dev server proxies `/api` and `/healthz` to PICLOUD_API
    (default `http://127.0.0.1:18080` to match the picloud bind
    override). Port configurable via PICLOUD_DASHBOARD_PORT.
  * `src/lib/api.ts` is a thin typed client over the control-plane
    paths; the scripts placeholder route shows the "load → error →
    list" shape so the missing-API state is informative, not blank.
  * SSR disabled at the layout level: the build is a pure SPA, no
    Node runtime is required at serve time.
  * `npm run check` and `npm run build` both green; `npm audit`
    clean (cookie override pins past the SvelteKit transitive
    advisory that doesn't apply in SPA mode).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashboard/src/routes/+layout.svelte

@@ -0,0 +1,65 @@
+<script lang="ts">
+	let { children } = $props();
+</script>
+
+<div class="shell">
+	<header>
+		<a href="/" class="brand">PiCloud</a>
+		<nav>
+			<a href="/">Scripts</a>
+		</nav>
+	</header>
+	<main>
+		{@render children?.()}
+	</main>
+</div>
+
+<style>
+	:global(html, body) {
+		margin: 0;
+		font-family: ui-sans-serif, system-ui, -apple-system, 'Segoe UI', sans-serif;
+		background: #0f172a;
+		color: #e2e8f0;
+	}
+
+	.shell {
+		min-height: 100vh;
+		display: flex;
+		flex-direction: column;
+	}
+
+	header {
+		display: flex;
+		align-items: center;
+		gap: 2rem;
+		padding: 1rem 2rem;
+		border-bottom: 1px solid #1e293b;
+		background: #0b1220;
+	}
+
+	.brand {
+		font-weight: 700;
+		font-size: 1.125rem;
+		color: #38bdf8;
+		text-decoration: none;
+	}
+
+	nav a {
+		color: #94a3b8;
+		text-decoration: none;
+		font-size: 0.9rem;
+	}
+
+	nav a:hover {
+		color: #e2e8f0;
+	}
+
+	main {
+		flex: 1;
+		padding: 2rem;
+		max-width: 64rem;
+		width: 100%;
+		margin: 0 auto;
+		box-sizing: border-box;
+	}
+</style>