Add shimmer loading state for favorites on Home

[IgnacioLD]

Closes #3

What

Favorites on the Home screen used to flash the empty state and then pop in, because the stops catalog emits an empty list before its first network fetch resolves. This shows shimmer placeholder cards while favorites are pending instead.

Changes

• Shimmer loading: new FavoriteStopCardSkeleton that mirrors the real FavoriteStopCard layout; HomeUiState.Ready gains an isLoading flag (true when the user has favorited stops but they haven't resolved yet), and HomeScreen renders the skeleton during that window.
• Fix pull-to-refresh getting stuck:
  • The polling loop consumed refresh signals without breaking its wait, so arrivals only re-fetched on the 30s cadence. The loop now breaks on signal for an immediate fetch (arrivals + alerts).
  • AnimatedContent was keyed on the whole state, recreating PullToRefreshBox mid-gesture. Keyed it on the state kind (contentKey) instead.
  • The spinner now dismisses on a fixed timer instead of waiting on data arrival (which can be deduped by distinctUntilChanged and leave it stuck).
• Fix bottom inset: the nested Home Scaffold re-applied the system navigation-bar inset even though the host Scaffold already insets above the bottom bar, cutting into the list. Set contentWindowInsets = WindowInsets(0).

Tested

Built and ran on a physical device: favorites shimmer while loading, pull-to-refresh updates instantly and the spinner dismisses promptly, and the list bottom is no longer cut off.