Fix #960: scope subscription counts, import/export to active account

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

Yattee/Views/Channel/ChannelView.swift

@@ -2097,9 +2097,13 @@ struct ChannelView: View {
         isLoading = true
         errorMessage = nil
 
-        // Load subscription state
+        // Load subscription state: optimistic from the local store, then
+        // corrected by the active provider (server accounts differ from local)
         subscription = appEnvironment.dataManager.subscription(for: channelID)
         isSubscribed = subscription != nil
+        Task {
+            isSubscribed = await appEnvironment.subscriptionService.isSubscribed(to: channelID)
+        }
 
         // Load cached header data for immediate display
         cachedHeader = CachedChannelData.load(for: channelID, using: appEnvironment.dataManager)
@@ -2212,6 +2216,9 @@ struct ChannelView: View {
                 // Check subscription status using extracted channel ID
                 subscription = appEnvironment.dataManager.subscription(for: fetchedChannel.id.channelID)
                 isSubscribed = subscription != nil
+                Task {
+                    isSubscribed = await appEnvironment.subscriptionService.isSubscribed(to: fetchedChannel.id.channelID)
+                }
             }
         } catch let error as APIError {
             await MainActor.run {
@@ -2247,6 +2254,11 @@ struct ChannelView: View {
         let effectiveChannelID = channel?.id.channelID ?? channelID
         subscription = appEnvironment?.dataManager.subscription(for: effectiveChannelID)
         isSubscribed = subscription != nil
+        Task {
+            if let service = appEnvironment?.subscriptionService {
+                isSubscribed = await service.isSubscribed(to: effectiveChannelID)
+            }
+        }
     }
 
     private func loadMoreVideos() async {