Performance: Navigation delay / popstate lag when returning to main screen with multiple calendars

[iNikAnn]

Describe the bug

There is a noticeable delay/lag when navigating back from the menu to the main route.

To Reproduce

1. Enable Power Saving mode on a PC or use a lower-end/older device.
2. Create multiple habits.
3. Open the menu.
4. Click Back to return to the main screen.
5. Notice the delay before the menu exit animation begins.

Expected behavior

The navigation and exit animation should start immediately without a blocking delay. Heavy calendar calculations should be optimized or deferred.

Additional context

In the browser console it triggers a warning: [Violation] 'popstate' handler took ***ms. The lag scales with the number of habits. We need to find a solution, like lazy loading calendars or delaying their render until the animation finishes.