fix(server-storage): apply a caller's edits to the cache as it is now

ServerUserDataStorage remembers the map getItem handed out. When the
cache changed since (another tab saved, a re-read), a write applies only
the entries the caller changed to the current cache, so a map read
before another tab's save no longer reads as deleting that save. The
cache moves into UserDataCache.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture/pwa-self-hosted.md

@@ -203,7 +203,9 @@ settings and guards read them in every build.
   the four synced namespaces are cached under the `iptvnator-server:` key
   prefix (the legacy unprefixed keys are never written in server mode) and
   each change sends only the playlist entries that differ, debounced, with a
-  `keepalive` flush when the page hides. Batches go out one after another
+  `keepalive` flush when the page hides. A change is compared with the map
+  the caller read: when the cache changed since (another tab, a re-read),
+  only the entries the caller changed are applied to it. Batches go out one after another
   (the page-hide flush waits for a batch still travelling), a failed write,
   including a refused `keepalive` one, stays queued for the next flush, and
   every unconfirmed write (queued or travelling) is saved in `localStorage`