feat: Enable Code Interpreter via URL parameter

Adds the ability to enable the Code Interpreter feature by appending
`code-interpreter=true` to the chat URL.

The `Chat.svelte` component's `initNewChat` function has been updated
to parse this URL parameter. If present, it sets the
`codeInterpreterEnabled` state variable to true.

This change leverages the existing mechanisms for:
- Saving and restoring `codeInterpreterEnabled` state to/from localStorage.
- Displaying and toggling the Code Interpreter state via the existing UI
  button in the `MessageInput` component.

No new UI elements were added as the existing UI correctly reflects
the feature's state.

src/lib/components/chat/Chat.svelte

@@ -813,6 +813,10 @@
 			imageGenerationEnabled = true;
 		}
 
+		if ($page.url.searchParams.get('code-interpreter') === 'true') {
+			codeInterpreterEnabled = true;
+		}
+
 		if ($page.url.searchParams.get('tools')) {
 			selectedToolIds = $page.url.searchParams
 				.get('tools')