Fix: Patch structuredClone in dev to correctly clone $state proxies

.changeset/weak-flowers-sit.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+patch structuredClone in dev to error on $state proxies

documentation/docs/98-reference/.generated/client-errors.md

@@ -129,3 +129,9 @@ Reading state that was created inside the same derived is forbidden. Consider us
 ```
 Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
 ```
+
+### structured_clone_state_proxy
+
+```
+To correctly clone a `$state` proxy, use `$state.snapshot` instead of `structuredClone`
+```

packages/svelte/messages/client-errors/errors.md

@@ -83,3 +83,7 @@
 ## state_unsafe_mutation
 
 > Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
+
+## structured_clone_state_proxy
+
+> To correctly clone a `$state` proxy, use `$state.snapshot` instead of `structuredClone`

packages/svelte/src/internal/client/errors.js

@@ -335,4 +335,19 @@ export function state_unsafe_mutation() {
 	} else {
 		throw new Error(`https://svelte.dev/e/state_unsafe_mutation`);
 	}
+}
+
+/**
+ * To correctly clone a `$state` proxy, use `$state.snapshot` instead of `structuredClone`
+ * @returns {never}
+ */
+export function structured_clone_state_proxy() {
+	if (DEV) {
+		const error = new Error(`structured_clone_state_proxy\nTo correctly clone a \`$state\` proxy, use \`$state.snapshot\` instead of \`structuredClone\`\nhttps://svelte.dev/e/structured_clone_state_proxy`);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/structured_clone_state_proxy`);
+	}
 }