Server-side savegames: Rust service with a shared savegame list in the menu, fallback to files

- server/: small axum service storing savegames as files (1 MB each, 500 MB total, rate limit)
- docker compose: second container with a volume; nginx forwards /api/
- JS plugin: asynchronous HTTP requests polled by the game
- Save/Load open a list page (name field, new, overwrite, load, delete with confirmation)
- Without a reachable service the game keeps using savegame files and the browser autosave

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

nginx.conf

@@ -7,4 +7,13 @@ server {
     location / {
         try_files $uri $uri/ =404;
     }
+    # savegame service (server/). Resolved at request time, so the game still starts without it;
+    # the game then falls back to savegame files.
+    location /api/ {
+        resolver 127.0.0.11 valid=10s ipv6=off;
+        set $saves http://saves:8080;
+        proxy_pass $saves;
+        client_max_body_size 2m;
+        proxy_read_timeout 15s;
+    }
 }