ImportSubscriptionsView wrote every imported channel straight into the
local SwiftData store via dataManager.subscribe(to:), regardless of the
selected account context. Imports now go through
SubscriptionService.subscribe(to:in:) aimed at a destination the user
picks — the local store or any logged-in account that supports
subscriptions, minus the source instance's own account, the same roster
the subscribe menus offer.
The already-subscribed marks are asked per destination through a new
SubscriptionService.subscribedChannelIDs(in:) helper (cold server
destinations fetch their list once, generation-fenced like the other
reads) and reload when the destination changes. Individual adds show a
per-row spinner while the server write is in flight, alreadySubscribed
counts as success, and Add All runs serially with an added/failed tally.
Also drop the "Invidious" wording from the import empty state — the
view serves Piped sources too.
101880d35f