feat(embed): standalone widget route /widget/<collection>/<slug>

Renders a single CMS entry as a block in isolation — no header, footer, or
preview banner. Lets us share a deep link to a single widget (e.g. the live
strommix gauge) for embedding or quick previews without exposing the rest of
the page.

- New BlockRenderer.svelte: switch over `_type` shared with ContentRows.
- Generic `getEntryBySlug(collection, slug, options)` in cms.ts so any
  collection (live_strommix, deadline_banner, image, ...) is fetchable.
- Route group `(embed)/+layout@.svelte` resets the layout chain so no
  Header/Footer/TopBanner inherit. `x-robots-tag: noindex`.
- Optional `?preview=<token>` for drafts. Cache: short s-maxage for live,
  no-store for preview.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/routes/(embed)/widget/[collection]/[slug]/+page.server.ts

@@ -0,0 +1,32 @@
+import { error } from "@sveltejs/kit";
+import { getEntryBySlug } from "$lib/cms";
+import type { PageServerLoad } from "./$types";
+
+export const load: PageServerLoad = async ({ params, url, setHeaders }) => {
+  const { collection, slug } = params;
+  const preview = url.searchParams.get("preview") ?? undefined;
+  const locale = url.searchParams.get("_locale") ?? undefined;
+
+  const entry = await getEntryBySlug<Record<string, unknown>>(collection, slug, {
+    resolve: ["all"],
+    preview,
+    locale,
+  });
+  if (!entry) throw error(404, `Entry not found: ${collection}/${slug}`);
+
+  // _type is needed by BlockRenderer's switch. Server backfills it from the
+  // route param so the renderer sees the same shape as inside a page row.
+  if (typeof entry._type !== "string") {
+    (entry as Record<string, unknown>)._type = collection;
+  }
+
+  // Embeds: skip search engines, mild caching for public reads.
+  setHeaders({
+    "x-robots-tag": "noindex",
+    "cache-control": preview
+      ? "private, no-store"
+      : "public, s-maxage=60, stale-while-revalidate=300",
+  });
+
+  return { collection, slug, entry, preview: !!preview };
+};

src/routes/(embed)/widget/[collection]/[slug]/+page.svelte

@@ -0,0 +1,23 @@
+<script lang="ts">
+  import BlockRenderer from "$lib/components/BlockRenderer.svelte";
+  import type { ResolvedBlock } from "$lib/block-types";
+  import type { PageData } from "./$types";
+
+  let { data }: { data: PageData } = $props();
+  const block = $derived(data.entry as ResolvedBlock);
+</script>
+
+<svelte:head>
+  <title>{data.collection} / {data.slug}</title>
+  <meta name="robots" content="noindex" />
+</svelte:head>
+
+<div class="widget-shell">
+  <BlockRenderer {block} />
+</div>
+
+<style>
+  .widget-shell {
+    padding: 1rem;
+  }
+</style>