feat(#1339): Deploy Nexus to proper URL for preview

ES module imports fail via file:// or raw Forge URLs (CORS +
missing Content-Type headers). boot.js detects this and warns users.

Three deployment options:
  - ./preview.sh — local Python server, correct MIME types
  - docker compose up nexus-preview — nginx on :8080 + WS proxy
  - Push to main — GitHub Pages auto-deploy

Fixes #1339

docker-compose.yml

@@ -7,9 +7,28 @@ services:
     restart: unless-stopped
     ports:
       - "8765:8765"
+
   nexus-staging:
     build: .
     container_name: nexus-staging
     restart: unless-stopped
     ports:
-      - "8766:8765"
+      - "8766:8765"
+
+  nexus-backend:
+    build: .
+    container_name: nexus-backend
+    restart: unless-stopped
+    expose:
+      - "8765"
+
+  nexus-preview:
+    build:
+      context: .
+      dockerfile: Dockerfile.preview
+    container_name: nexus-preview
+    restart: unless-stopped
+    ports:
+      - "8080:8080"
+    depends_on:
+      - nexus-backend