fix(server-storage): page-hide keepalive writes report an outage too

sendKeepalive() turned a failed fetch, a timeout or a gateway's
502/503/504 into a failed outcome without telling the status, so the
instance stayed verified: when the hidden tab came back, its refresh
skipped the /instance check and resent the queued writes, which could
reach a replacement database behind the same address. Like requests, a
keepalive write now reports these as offline, so the instance is
checked before anything goes out again.

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

docs/architecture/pwa-self-hosted.md

@@ -348,8 +348,9 @@ settings and guards read them in every build.
   trouble. A page started from memory first checks that the instance at the
   address is the remembered one (`ensureVerified`), and reloads if not,
   before sign-in sends the token. So does a page that went offline (any
-  backend call can tell: the storage API, `HttpClient` calls through the
-  interceptor and `PwaService`'s raw fetches report no answer, or a
+  backend call can tell: the storage API with its page-hide `keepalive`
+  writes, `HttpClient` calls through the interceptor and `PwaService`'s raw
+  fetches report no answer, or a
   gateway's 502/503/504 that is not the backend's own JSON, as offline)
   before its next read: until then it sends neither the token nor writes (a
   replaced volume at the same address is another database), and it checks