Slice 4 of the smooth-loading pass. Adds CachedHistorySnapshot
(schema 4) and rewires _historyProvider through cacheFirst, mirroring
the homeProvider pattern: yield the last cached page immediately on
subscribe so the tab paints from disk on cold open, then SWR-refresh
in the background to surface fresh plays.
Also enables basic offline scrollback — the most recent History page
survives both app restart and connectivity loss.
JSON blob storage (vs columnar) because the page is small, always
read whole, and HistoryPage.fromJson already accepts the wire shape,
so server-side field additions don't force a migration.
History delta sync via library_changes is out of scope here; the next
visit's SWR pull is the source of freshness for now.
bvandeusen
395a6efb26