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>

docker-compose.yml

@@ -6,3 +6,14 @@ services:
     ports:
       - "18093:80"
     restart: unless-stopped
+  # savegame service, only reachable through the dungeoncraft container (/api/)
+  saves:
+    build: ./server
+    image: dungeoncraft-saves:latest
+    container_name: dungeoncraft-saves
+    volumes:
+      - savegames:/data
+    restart: unless-stopped
+
+volumes:
+  savegames: