feat(admin): set /admin base path for reverse proxy deployment

Configure Vite base, Vue Router history, axios baseURL and container
Nginx to serve the admin panel under /admin/ path prefix.

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docker/nginx/admin.conf

@@ -9,9 +9,8 @@ server {
     gzip on;
     gzip_types text/plain text/css application/json application/javascript text/xml application/xml;
 
-    location /api/ {
-        set $api_upstream api:3000;
-        proxy_pass http://$api_upstream;
+    location /admin/api/ {
+        proxy_pass http://api:3000/api/;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
@@ -19,7 +18,11 @@ server {
         proxy_set_header X-Forwarded-Proto $scheme;
     }
 
+    location /admin/ {
+        try_files $uri $uri/ /admin/index.html;
+    }
+
     location / {
-        try_files $uri $uri/ /index.html;
+        return 301 /admin/;
     }
 }