feat: live staging with auto-refresh on main push (Fixes #33)

- index.html: polls Gitea API every 30s for new commits on main;
  shows countdown banner and auto-reloads when SHA changes
- Dockerfile + docker-compose.yml: serve the site via nginx;
  main on :4200, staging on :4201
- deploy.sh: rebuild and restart a named service
- .gitea/workflows/deploy.yml: SSH-deploy to host on every push to main
  (requires DEPLOY_HOST / DEPLOY_USER / DEPLOY_SSH_KEY repo secrets)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,20 @@
+version: "3.9"
+
+services:
+  nexus-main:
+    build: .
+    container_name: nexus-main
+    restart: unless-stopped
+    ports:
+      - "4200:80"
+    labels:
+      - "deployment=main"
+
+  nexus-staging:
+    build: .
+    container_name: nexus-staging
+    restart: unless-stopped
+    ports:
+      - "4201:80"
+    labels:
+      - "deployment=staging"