fix: nginx default page hijacks port 80 in Docker image

Two issues in Dockerfile.complete caused the nginx welcome page to appear
instead of the BiliNote UI:

1. /etc/nginx/sites-enabled/default had `listen 80 default_server` which
   took priority over the custom config in conf.d/
2. The nginx config proxied / to frontend:80, but the Dockerfile sed
   replaced it with 127.0.0.1:8080 where no service was running. The
   frontend is built as static files in /usr/share/nginx/html/.

Fix:
- Remove /etc/nginx/sites-enabled/default in Dockerfile
- Change location / to serve static files directly instead of proxying
- Remove the frontend proxy sed (no longer needed)

nginx/default.conf

@@ -2,19 +2,18 @@ server {
   listen 80;
   client_max_body_size 10G;
 
-  # gzip 压缩
   gzip on;
   gzip_vary on;
   gzip_min_length 1024;
   gzip_proxied any;
   gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript image/svg+xml;
 
-  # 所有非 /api 请求全部代理给 frontend 容器
   location / {
-    proxy_pass http://frontend:80;
+    root /usr/share/nginx/html;
+    index index.html;
+    try_files $uri $uri/ /index.html;
   }
 
-  # 所有 /api 请求代理给 backend 容器
   location /api/ {
     proxy_pass http://backend:8483;
     proxy_set_header Host $host;