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>

www/dc_plugin.js

@@ -1,9 +1,12 @@
-// DungeonCraft browser plugin for miniquad: storage, file download/upload, save-on-leave, audio output.
+// DungeonCraft browser plugin for miniquad: storage, file download/upload, save-on-leave,
+// HTTP requests (savegame service), audio output.
 (function () {
     "use strict";
     var pending = null;      // value handed over by dc_storage_get
     var file = null;         // bytes of a file chosen via the file picker
     var cache = {};          // key -> string, persisted to localStorage when the page is hidden/closed
+    var http = {};           // request id -> { done, status, body }
+    var httpNext = 1;
 
     function bytes(ptr, len) {
         return new Uint8Array(wasm_memory.buffer, ptr, len);
@@ -34,8 +37,59 @@
     });
     document.addEventListener("DOMContentLoaded", function () { document.body.appendChild(input); });
 
+    // Asynchronous HTTP request; the result is polled by the game.
+    // status 0 = network error (e.g. no service, or the page was opened as a local file)
+    function httpStart(method, url, body) {
+        var id = httpNext++;
+        var req = { done: false, status: 0, body: new Uint8Array(0) };
+        http[id] = req;
+        var ctrl = typeof AbortController !== "undefined" ? new AbortController() : null;
+        var timer = setTimeout(function () { if (ctrl) ctrl.abort(); }, 15000);
+        try {
+            var opts = { method: method, cache: "no-store", headers: {} };
+            if (ctrl) opts.signal = ctrl.signal;
+            if (body !== null) { opts.body = body; opts.headers["Content-Type"] = "application/json"; }
+            fetch(url, opts).then(function (r) {
+                req.status = r.status;
+                return r.arrayBuffer();
+            }).then(function (buf) {
+                req.body = new Uint8Array(buf);
+                req.done = true;
+                clearTimeout(timer);
+            }).catch(function () {
+                req.status = 0;
+                req.done = true;
+                clearTimeout(timer);
+            });
+        } catch (e) {
+            req.done = true;
+        }
+        return id;
+    }
+
     function register(importObject) {
         var env = importObject.env;
+        env.dc_http = function (mp, ml, up, ul, bp, bl) {
+            var body = bl > 0 ? bytes(bp, bl).slice() : null;
+            return httpStart(str(mp, ml), str(up, ul), body);
+        };
+        // -2 = still running, otherwise the HTTP status (0 = network error)
+        env.dc_http_poll = function (id) {
+            var r = http[id];
+            if (!r) return 0;
+            return r.done ? r.status : -2;
+        };
+        env.dc_http_len = function (id) {
+            var r = http[id];
+            return r ? r.body.length : 0;
+        };
+        // copies the response body and forgets the request
+        env.dc_http_take = function (id, ptr) {
+            var r = http[id];
+            if (!r) return;
+            bytes(ptr, r.body.length).set(r.body);
+            delete http[id];
+        };
         env.dc_storage_get = function (kp, kl) {
             var v = null;
             try { v = window.localStorage.getItem(str(kp, kl)); } catch (e) { }