On tvOS, ContentUnavailableView inside a Group doesn't expand to fill
available space — it sizes to content and aligns top-leading. Add
.frame(maxWidth: .infinity, maxHeight: .infinity) to all instances
so they center correctly in their parent containers.
The filter strip was passing the Invidious instance URL as serverURL to
AvatarURLBuilder, which built a Yattee Server-style /avatar/ path that
doesn't exist on Invidious. Now passes the actual Yattee Server URL
(matching SubscriptionsView pattern) and enriches channels from
CachedChannelData as a fallback when the API doesn't return thumbnails.
When a cancelled load task fell through to `isLoading = false`, it
created a 1-frame gap where the empty view rendered before the
replacement task set `isLoading` back to `true`. Return early on
cancellation so the surviving task controls loading state.
Move .refreshable from the outer GeometryReader onto the ScrollView
itself so SwiftUI can properly coordinate the scroll offset bounce-back.
The ScrollView was inside an .overlay() which doesn't participate in
the parent's layout system, breaking the offset reset.
Closes#917
