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/lib/cms.ts

@@ -625,6 +625,42 @@ export async function getPostBySlug(
   return cached("post", key, fetchFn);
 }
 
+/**
+ * Generic single-entry fetch by `(collection, slug)`. Used by the embed/widget
+ * route that renders a single block standalone.
+ *
+ * Resolves nested references via `_resolve=all` by default so block components
+ * see the same data shape as inside a page row.
+ */
+export async function getEntryBySlug<T = Record<string, unknown>>(
+  collection: string,
+  slug: string,
+  options?: ContentGetOptions,
+): Promise<T | null> {
+  const resolveKey = options?.resolve?.join(",") ?? "all";
+  const fieldsKey = options?.fields?.join(",") ?? "";
+  const depthKey = options?.depth ?? "";
+  const key = `entry:${collection}:${slug}:${options?.locale ?? ""}:${resolveKey}:${depthKey}:${fieldsKey}`;
+  const fetchFn = async (): Promise<T | null> => {
+    const base = getBaseUrl();
+    const u = new URL(`${base}/api/content/${encodeURIComponent(collection)}/${encodeURIComponent(slug)}`);
+    if (options?.locale) u.searchParams.set("_locale", options.locale);
+    u.searchParams.set("_resolve", options?.resolve?.length ? options.resolve.join(",") : "all");
+    if (typeof options?.depth === "number") u.searchParams.set("_depth", String(options.depth));
+    if (options?.fields?.length) u.searchParams.set("_fields", options.fields.join(","));
+    if (options?.preview) u.searchParams.set("preview", options.preview);
+    if (options?.previewDraft) u.searchParams.set("preview_draft", options.previewDraft);
+    const res = await cmsFetch(u.toString());
+    if (res.status === 404) return null;
+    if (!res.ok) {
+      throw new Error(`RustyCMS getEntry failed: ${res.status} for ${collection}/${slug}`);
+    }
+    return (await res.json()) as T;
+  };
+  if (options?.preview || options?.previewDraft) return fetchFn();
+  return cached(collection, key, fetchFn);
+}
+
 /** Für Post-URLs wird slug bevorzugt (nicht _slug). Führender Slash wird ignoriert. */
 export async function getPostSlugs(): Promise<string[]> {
   const items = await getPosts();