feat(templates): aspect-ratio picker drives which variant is built

The detail page now loads a template's real published aspect variants (16:9/1:1/9:16)
from the content container and the preview chips select among them. Build now copies
the SELECTED variant's scene graph (passes that variant's content project UUID), not a
default. Selection is lifted to TemplateDetailContent and shared by the preview picker
and the build button; the preview box reflects the chosen aspect.

Verified on insta-promo (16:9 + a duplicated 1:1 variant): both chips render, and
building 1:1 copies the 1:1 project's scenes (1 scene, 6 fields).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/[locale]/templates/[id]/page.tsx

@@ -2,13 +2,16 @@ import type { Metadata } from "next";
 import { notFound } from "next/navigation";
 
 import { TemplateDetailContent } from "@/components/templates/TemplateDetailContent";
-import { fetchProject } from "@/lib/admin-api";
+import { fetchProject, fetchTemplateVariants } from "@/lib/admin-api";
 import {
   adminProjectToCatalogTemplate,
   VIDEO_TEMPLATES_CATALOG,
+  type TemplateDetailAspectRatio,
   type VideoCatalogTemplate,
 } from "@/lib/video-templates-catalog";
 
+const SUPPORTED_ASPECTS = new Set<TemplateDetailAspectRatio>(["16:9", "1:1", "9:16"]);
+
 interface TemplateDetailPageProps {
   params: Promise<{ id: string }>;
 }
@@ -20,7 +23,13 @@ interface TemplateDetailPageProps {
  */
 async function resolveTemplate(id: string): Promise<VideoCatalogTemplate | null> {
   const admin = await fetchProject(id);
-  if (admin) return adminProjectToCatalogTemplate(admin);
+  if (admin) {
+    const base = adminProjectToCatalogTemplate(admin);
+    const variants = (await fetchTemplateVariants(id))
+      .filter((v) => SUPPORTED_ASPECTS.has(v.aspect as TemplateDetailAspectRatio))
+      .map((v) => ({ aspect: v.aspect as TemplateDetailAspectRatio, projectId: v.projectId }));
+    return { ...base, variants };
+  }
   return VIDEO_TEMPLATES_CATALOG.find((item) => item.id === id) ?? null;
 }