When an Invidious/Piped account is active, several views read the local
SwiftData subscription store instead of the account, so counts
contradicted each other (Home tile said 10 while the Channels list
showed the server's 2):
- Home Channels tile now uses the provider's count via new
SubscriptionService.cachedSubscriptionCount (fetched once in the
background when the server cache isn't populated yet)
- Export footer and export content use the active account's list;
export runs async with a spinner and surfaces fetch errors as a toast
- CSV/OPML import routes through SubscriptionService.importSubscriptions,
subscribing on the server for server accounts instead of silently
writing to the invisible local store
- ChannelView subscribe-state is corrected via the provider after the
optimistic local-store read; unused *Sync write helpers removed
- Server-account subscribe/unsubscribe/import now post
subscriptionsDidChange so other views refresh
- New "Delete Local Subscription Data" section in Subscriptions
settings (visible with a server account) clears the local store and
queues CloudKit deletions so iCloud doesn't restore it
