Move header loading animation to CLI update checks

[brianlovin]

Summary

• Header loading spinner now only shows when checking for CLI updates
• Removed redundant header loading from story fetching (full-screen loading bar already exists)
• Header loading is now exclusively used during background update checks

Test plan

• Run the app and verify no loading indicator in header during initial story fetch
• The full-screen loading bar should appear instead
• Header loading spinner should appear briefly when checking for updates in the background