feat(store): rely on the delta feed instead of full snapshots once a sync cursor exists

The sync engine added an incremental channel, but the app kept fetching full
snapshots next to it: both prefetch hooks, a recount after every pull, every
loaded page of the entry lists, and whole-list refetches after unsubscribing or
importing.

- unread counters keep confirmed counts under the local transactions; the change
  log moves them by the flips and new unread rows the server reports, and an
  acknowledged transaction drops its prediction once the engine reached its
  sync id
- usePrefetchSubscription, usePrefetchUnread, pull to refresh, the refresh
  button, OPML import, unsubscribe and the mobile background task go through the
  engine and only fall back to the full requests when it cannot help
- snapshots remain as calibration: unread hourly, subscriptions daily, plus a
  throttled recount when the list on screen disagrees after a delta pull
- new entries fetch only the first page of the lists on screen and merge it in
  front of the loaded pages; a refresh trims the infinite query to one page
- mobile refreshes subscriptions through the delta feed after an OPML import

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>