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>

src/platform.rs

@@ -1,6 +1,13 @@
 // Browser integration via our own miniquad JS plugin (www/dc_plugin.js):
 // persistent key/value storage (localStorage), a cache that is written to storage when the
-// page is hidden or closed, file download and file upload. Native builds use simple fallbacks.
+// page is hidden or closed, file download, file upload and HTTP requests.
+// Native builds use simple fallbacks.
+
+// Result of an HTTP request: status 0 means the service could not be reached
+pub struct HttpResult {
+    pub status: i32,
+    pub body: Vec<u8>,
+}
 
 #[cfg(target_arch = "wasm32")]
 mod imp {
@@ -13,6 +20,27 @@ mod imp {
         fn dc_open_file();
         fn dc_file_poll() -> i32;
         fn dc_file_take(dst: *mut u8);
+        fn dc_http(m: *const u8, ml: u32, u: *const u8, ul: u32, b: *const u8, bl: u32) -> u32;
+        fn dc_http_poll(id: u32) -> i32;
+        fn dc_http_len(id: u32) -> u32;
+        fn dc_http_take(id: u32, dst: *mut u8);
+    }
+
+    // starts a request; poll the returned id with http_poll()
+    pub fn http(method: &str, url: &str, body: &[u8]) -> u32 {
+        unsafe { dc_http(method.as_ptr(), method.len() as u32, url.as_ptr(), url.len() as u32, body.as_ptr(), body.len() as u32) }
+    }
+
+    pub fn http_poll(id: u32) -> Option<super::HttpResult> {
+        unsafe {
+            let status = dc_http_poll(id);
+            if status == -2 {
+                return None;
+            }
+            let mut body = vec![0u8; dc_http_len(id) as usize];
+            dc_http_take(id, body.as_mut_ptr());
+            Some(super::HttpResult { status, body })
+        }
     }
 
     pub fn storage_get(key: &str) -> Option<String> {
@@ -81,6 +109,13 @@ mod imp {
     pub fn take_file() -> Option<Vec<u8>> {
         None
     }
+    // no savegame service in native builds: every request fails like an unreachable server
+    pub fn http(_method: &str, _url: &str, _body: &[u8]) -> u32 {
+        0
+    }
+    pub fn http_poll(_id: u32) -> Option<super::HttpResult> {
+        Some(super::HttpResult { status: 0, body: Vec::new() })
+    }
 }
 
 pub use imp::*;