Storyteller.ai API integration (#49)

This PR integrates asset retrieval from the Storyteller.ai back-end API,
and begins implementing long-term scene saving and storage.

### Media retrieval
Instead of fetching raw asset URLs, going forward, media files from
remote sources (e.g., in URL parameters) should be specified with a path
like the following:
```
remote://<token>.<filetype>
```
where
* `token` is a media token (e.g., "m_70b77neq9jsgmy5v31mekzchcbd223")
* `filetype` is a file extension appropriate for the type of file being
  fetched (e.g., "gltf")

The `remote` protocol will be recognized by a [custom asset
reader](https://github.com/storytold/storyteller-bevy/blob/70726db111f6ccafd47feb055c5546540fef1786/studio/src/remote/reader.rs)
which will retrieve the file by calling a Storyteller API endpoint.

### Scene storage
Additionally, this PR begins laying the groundwork for
storing/retrieving Bevy scenes to/from the backend instead of solely
in-memory. This work is still incomplete -- as of this PR, a button has
been added to the web UI allowing the user to download the scene
file locally for testing/debugging purposes.

### Test URL
Media retrieval from the backend can be tested by [visiting this
URL](https://deploy-preview-49--storyteller-studio.netlify.app/?mode=viewer&mixamo=remote://m_70b77neq9jsgmy5v31mekzchcbd223.gltf)
in Chrome with the `--disable-web-security` flag (to bypass CORS errors)

studio/src/persistance/mod.rs

@@ -10,6 +10,13 @@ impl Plugin for PersistancePlugin {
             EditorTabName::Other("Persistance".to_string()),
             DebugPersistanceTab::default(),
         );
+
+        #[cfg(feature = "wasm")]
+        app.add_systems(Update, wasm::forward_save_scene_events)
+            .add_systems(
+                OnEnter(SaveState::Idle),
+                wasm::on_scene_saved.map(bevy::utils::warn),
+            );
     }
 }
 
@@ -43,3 +50,64 @@ impl EditorTab for DebugPersistanceTab {
         "Persistance".into()
     }
 }
+
+#[cfg(feature = "wasm")]
+mod wasm {
+    use bevy::{prelude::*, window::PrimaryWindow};
+    use space_editor::prelude::{EditorEvent, EditorPrefabPath, PrefabMemoryCache};
+    use wasm_bindgen::prelude::*;
+
+    use crate::wasm::{self, StaticBuffer, WasmMessageBuffer};
+
+    static SAVE_SCENE: WasmMessageBuffer<()> = WasmMessageBuffer::new();
+
+    #[wasm_bindgen(js_name = saveScene)]
+    pub fn save_scene() {
+        SAVE_SCENE.write_message(());
+    }
+
+    pub(super) fn forward_save_scene_events(mut ew_editor_events: EventWriter<EditorEvent>) {
+        if SAVE_SCENE.take_message().is_some() {
+            ew_editor_events.send(EditorEvent::Save(EditorPrefabPath::MemoryCahce));
+        }
+    }
+
+    #[wasm_bindgen(typescript_custom_section)]
+    const SCENE_SAVED_EVENT: &str = r#"
+export interface SceneSavedEvent extends CustomEvent {
+    type: "scene-saved";
+    detail: string;
+}
+
+declare global {
+    export interface GlobalEventHandlersEventMap {
+        "scene-saved": SceneSavedEvent;
+    }
+}
+    "#;
+
+    pub(super) fn on_scene_saved(
+        r_scene_cache: Res<PrefabMemoryCache>,
+        r_type_registry: Res<AppTypeRegistry>,
+        ra_scenes: Res<Assets<DynamicScene>>,
+        q_window: Query<&Window, With<PrimaryWindow>>,
+    ) -> Result<(), String> {
+        let handle = r_scene_cache
+            .scene
+            .as_ref()
+            .ok_or_else(|| "Scene cache is empty!".to_string())?;
+
+        let serialized = match ra_scenes.get(handle) {
+            Some(scene) => scene
+                .serialize_ron(&r_type_registry)
+                .map_err(|err| format!("{err}")),
+            None => Err("No scene found for cached handle!".into()),
+        }?;
+
+        let Ok(window) = q_window.get_single() else {
+            return Ok(());
+        };
+
+        wasm::dispatch_to_js(window, "scene-saved", &serialized)
+    }
+}