Add cloud download icon and improve update UI handling

Update UI to use CloudDownload icon instead of RefreshCw
Improve error handling for update checks
Add better visual feedback for update states
Change GitHub link to RealManual repository

landing/index.html

@@ -1105,7 +1105,7 @@
           Скачать для macOS
         </a>
       </div>
-      <a href="https://github.com/spacesh" target="_blank" rel="noopener" class="cta-github">
+      <a href="https://git.realmanual.ru/realmanual/spaceshell" target="_blank" rel="noopener" class="cta-github">
         Исходники на GitHub →
       </a>
     </div>