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>

server/Dockerfile

@@ -0,0 +1,13 @@
+FROM rust:1-slim AS build
+WORKDIR /src
+COPY Cargo.toml Cargo.lock* ./
+RUN mkdir src && echo "fn main() {}" > src/main.rs && cargo build --release && rm -rf src
+COPY src src
+RUN touch src/main.rs && cargo build --release
+
+FROM debian:bookworm-slim
+COPY --from=build /src/target/release/dungeoncraft-saves /usr/local/bin/dungeoncraft-saves
+ENV SAVE_DIR=/data LISTEN=0.0.0.0:8080
+VOLUME /data
+EXPOSE 8080
+CMD ["dungeoncraft-saves"]