Title screen, pause menu, settings dialog and savegames

- title screen (continue/new/load/settings/help) and Esc pause menu
- settings: language & sound, camera, graphics, game, rebindable keys;
  stored in localStorage and in every savegame (always applied on load)
- savegames as JSON (.dcsave): download, load via file picker or drag & drop
- autosave to localStorage every 2 minutes and when the tab is hidden/closed
- configurable wall detail, light count, interface size, game speed, hints
- audio behind a cargo feature so native tests run (cargo test --no-default-features)
- savegame round-trip test

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

www/dc_plugin.js

@@ -1,7 +1,9 @@
-// DungeonCraft browser plugin for miniquad: localStorage access for the Rust code.
+// DungeonCraft browser plugin for miniquad: storage, file download/upload, save-on-leave.
 (function () {
     "use strict";
-    var pending = null;
+    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
 
     function bytes(ptr, len) {
         return new Uint8Array(wasm_memory.buffer, ptr, len);
@@ -9,23 +11,70 @@
     function str(ptr, len) {
         return new TextDecoder().decode(bytes(ptr, len));
     }
+    function flushCache() {
+        for (var k in cache) {
+            try { window.localStorage.setItem(k, cache[k]); } catch (e) { }
+        }
+    }
+    window.addEventListener("pagehide", flushCache);
+    document.addEventListener("visibilitychange", function () {
+        if (document.visibilityState === "hidden") flushCache();
+    });
+
+    // hidden <input type=file> for loading savegames
+    var input = document.createElement("input");
+    input.type = "file";
+    input.accept = ".dcsave,.json,application/json";
+    input.style.display = "none";
+    input.addEventListener("change", function () {
+        var f = input.files && input.files[0];
+        if (!f) return;
+        f.arrayBuffer().then(function (buf) { file = new Uint8Array(buf); });
+        input.value = "";
+    });
+    document.addEventListener("DOMContentLoaded", function () { document.body.appendChild(input); });
 
     function register(importObject) {
-        importObject.env.dc_storage_get = function (kp, kl) {
+        var env = importObject.env;
+        env.dc_storage_get = function (kp, kl) {
             var v = null;
             try { v = window.localStorage.getItem(str(kp, kl)); } catch (e) { }
             if (v === null) { pending = null; return -1; }
             pending = new TextEncoder().encode(v);
             return pending.length;
         };
-        importObject.env.dc_storage_take = function (ptr) {
+        env.dc_storage_take = function (ptr) {
             if (pending === null) return;
             bytes(ptr, pending.length).set(pending);
             pending = null;
         };
-        importObject.env.dc_storage_set = function (kp, kl, vp, vl) {
+        env.dc_storage_set = function (kp, kl, vp, vl) {
             try { window.localStorage.setItem(str(kp, kl), str(vp, vl)); } catch (e) { }
         };
+        env.dc_cache_set = function (kp, kl, vp, vl) {
+            cache[str(kp, kl)] = str(vp, vl);
+        };
+        env.dc_download = function (np, nl, dp, dl) {
+            var blob = new Blob([bytes(dp, dl).slice()], { type: "application/json" });
+            var a = document.createElement("a");
+            a.href = URL.createObjectURL(blob);
+            a.download = str(np, nl);
+            document.body.appendChild(a);
+            a.click();
+            setTimeout(function () { URL.revokeObjectURL(a.href); a.remove(); }, 1000);
+        };
+        env.dc_open_file = function () {
+            file = null;
+            input.click();
+        };
+        env.dc_file_poll = function () {
+            return file === null ? -1 : file.length;
+        };
+        env.dc_file_take = function (ptr) {
+            if (file === null) return;
+            bytes(ptr, file.length).set(file);
+            file = null;
+        };
     }
 
     miniquad_add_plugin({ register_plugin: register, version: 1, name: "dungeoncraft" });