new api_endpoint store + Config UI

ref #9

src/store.js

@@ -1,5 +1,10 @@
 import { writable } from 'svelte/store';
 
+const stored_api_endpoint = localStorage.getItem('api_endpoint')||"";
+export const api_endpoint = writable(stored_api_endpoint);
+api_endpoint.subscribe((value) => {
+	localStorage.setItem('api_endpoint', value);
+});
 const stored_apikey = localStorage.getItem('apikey');
 export const apikey = writable(stored_apikey);
 apikey.subscribe((value) => {