Keep single Save button, Show next sync, default to prod Khoj URL in Desktop app

- Make mutable syncing variable not a const
- Show next sync time to make users aware of data sync is automated
- Keep a single Save button to reduce confusion. It does what Save All
  previously did. Intent to manual sync should Save All
- Default to using app.khoj.dev as default Khoj URL to ease setup

src/interface/desktop/main.js

@@ -10,7 +10,7 @@ const {dialog} = require('electron');
 const cron = require('cron').CronJob;
 const axios = require('axios');
 
-const KHOJ_URL = 'http://127.0.0.1:42110'
+const KHOJ_URL = 'https://app.khoj.dev';
 
 const Store = require('electron-store');
 
@@ -67,7 +67,7 @@ const schema = {
     }
 };
 
-const syncing = false;
+let syncing = false;
 var state = {}
 const store = new Store({ schema });