fix(detail+docker): per-aspect template preview + Debian frontend base

- Template detail page now shows the render matching the SELECTED aspect (poster +
  preview video) instead of the 16:9 cover cropped into a 9:16/1:1 box. TemplateVariant
  carries per-aspect image/previewVideo; fetchTemplateVariants + the detail page wire them.
- AppShowcase3D ships a distinct preview video per aspect (seed PERASPECT_VIDEO).
- Frontend Dockerfile: Alpine -> node:20-slim (glibc). Fixes next-swc ("ld-linux..."
  load failure that broke `next build` once libc6-compat was removed) AND the original
  CI Alpine-CDN issue. Healthcheck switched to node (slim has no wget).

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

docker-compose.v2.yml

@@ -370,7 +370,7 @@ services:
       gateway:
         condition: service_healthy
     healthcheck:
-      test: ["CMD-SHELL", "wget -qO- http://127.0.0.1:3000 || exit 1"]
+      test: ["CMD", "node", "-e", "require('http').get('http://127.0.0.1:3000',r=>process.exit(r.statusCode<500?0:1)).on('error',()=>process.exit(1))"]
       interval: 30s
       timeout: 10s
       retries: 3