On iPad in regular horizontal size class, the Subscriptions view now
shows a channels column next to the feed (mirroring macOS/tvOS) instead
of the floating channel strip. iPhone and compact-width iPad keep the
existing strip.
- Renames MacSubscriptionsSidebarRow to SubscriptionsSidebarRow and
shares it across macOS and iOS.
- Uses a custom ScrollView-based sidebar on iPad to avoid iOS 26's
sidebar background extension bleeding into the selection highlight.
- Forces inline toolbar title on iPad regular so scrolling either
column behaves consistently.
