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>

src/platform.rs

@@ -1,5 +1,6 @@
 // Browser integration via our own miniquad JS plugin (www/dc_plugin.js):
-// persistent key/value storage (localStorage). Native builds use an in-memory fallback.
+// 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.
 
 #[cfg(target_arch = "wasm32")]
 mod imp {
@@ -7,6 +8,11 @@ mod imp {
         fn dc_storage_get(key: *const u8, key_len: u32) -> i32;
         fn dc_storage_take(dst: *mut u8);
         fn dc_storage_set(key: *const u8, key_len: u32, val: *const u8, val_len: u32);
+        fn dc_cache_set(key: *const u8, key_len: u32, val: *const u8, val_len: u32);
+        fn dc_download(name: *const u8, name_len: u32, data: *const u8, data_len: u32);
+        fn dc_open_file();
+        fn dc_file_poll() -> i32;
+        fn dc_file_take(dst: *mut u8);
     }
 
     pub fn storage_get(key: &str) -> Option<String> {
@@ -24,6 +30,32 @@ mod imp {
     pub fn storage_set(key: &str, val: &str) {
         unsafe { dc_storage_set(key.as_ptr(), key.len() as u32, val.as_ptr(), val.len() as u32) }
     }
+
+    // value is written to storage by JS when the page gets hidden or closed
+    pub fn cache_on_leave(key: &str, val: &str) {
+        unsafe { dc_cache_set(key.as_ptr(), key.len() as u32, val.as_ptr(), val.len() as u32) }
+    }
+
+    pub fn download(name: &str, data: &[u8]) {
+        unsafe { dc_download(name.as_ptr(), name.len() as u32, data.as_ptr(), data.len() as u32) }
+    }
+
+    // opens the browser file picker; poll with take_file()
+    pub fn open_file() {
+        unsafe { dc_open_file() }
+    }
+
+    pub fn take_file() -> Option<Vec<u8>> {
+        unsafe {
+            let n = dc_file_poll();
+            if n < 0 {
+                return None;
+            }
+            let mut buf = vec![0u8; n as usize];
+            dc_file_take(buf.as_mut_ptr());
+            Some(buf)
+        }
+    }
 }
 
 #[cfg(not(target_arch = "wasm32"))]
@@ -39,6 +71,16 @@ mod imp {
     pub fn storage_set(key: &str, val: &str) {
         STORE.with(|s| s.borrow_mut().insert(key.to_string(), val.to_string()));
     }
+    pub fn cache_on_leave(key: &str, val: &str) {
+        storage_set(key, val);
+    }
+    pub fn download(name: &str, data: &[u8]) {
+        let _ = std::fs::write(name, data);
+    }
+    pub fn open_file() {}
+    pub fn take_file() -> Option<Vec<u8>> {
+        None
+    }
 }
 
 pub use imp::*;