Sync scene selection with URL

src/main.js

@@ -14,6 +14,29 @@
   } = Matter;
 
   const { scenes = [], defaultSceneId } = window.PhysilinksScenes || {};
+  const getSceneById = (sceneId) =>
+    scenes.find((candidate) => candidate.id === sceneId) || null;
+
+  const getSceneIdFromUrl = () => {
+    try {
+      const params = new URLSearchParams(window.location.search);
+      const urlScene = params.get("scene");
+      return getSceneById(urlScene) ? urlScene : null;
+    } catch (err) {
+      return null;
+    }
+  };
+
+  const setSceneIdInUrl = (sceneId) => {
+    if (!sceneId) return;
+    try {
+      const url = new URL(window.location.href);
+      url.searchParams.set("scene", sceneId);
+      history.replaceState({}, "", `${url.pathname}${url.search}${url.hash}`);
+    } catch (err) {
+      // ignore history failures (blocked or unsupported)
+    }
+  };
 
   const config = {
     gravity: 1,
@@ -80,8 +103,11 @@
   let boundaries = [];
   let rotators = [];
   let oscillators = [];
-  let currentScene =
-    scenes.find((s) => s.id === defaultSceneId) || scenes[0] || null;
+  const initialSceneId =
+    getSceneIdFromUrl() ||
+    (getSceneById(defaultSceneId) ? defaultSceneId : null) ||
+    scenes[0]?.id;
+  let currentScene = getSceneById(initialSceneId) || scenes[0] || null;
 
   if (currentScene && currentScene.config) {
     Object.assign(config, currentScene.config);
@@ -131,9 +157,11 @@
   };
 
   const applyScene = (sceneId) => {
-    const next = scenes.find((s) => s.id === sceneId) || scenes[0];
+    const next = getSceneById(sceneId) || scenes[0];
+    if (!next) return;
     currentScene = next;
     ui.setSceneSelection(next.id);
+    setSceneIdInUrl(next.id);
     const prevRadius = config.ballRadius;
     Object.assign(config, next.config);
     config.link = { ...next.config.link };