fix(server-storage): offline recheck checks only the instance; sign-out always navigates

The recheck on going offline ran a full refresh: when /instance answered
but /user-state kept failing, each failed read set the page offline
again and started the next one at once, bypassing the retry interval.
It now checks the instance only; reads wait for the retry.

Sign-out navigates to sign-in itself, also when the page is signed out
already: a guard (unsaved Settings) may have cancelled the redirect the
first time, and the state change that drives it does not repeat.

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

docs/architecture/pwa-self-hosted.md

@@ -336,7 +336,10 @@ settings and guards read them in every build.
   before sign-in sends the token. So does a page that went 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
-  as soon as it goes offline, so a blip costs no retry interval. Both turn
+  the instance as soon as it goes offline, so a blip costs no retry
+  interval (only the instance: a read that keeps failing waits for the
+  retry). Sign-out always navigates to sign-in, also when a guard kept the
+  page the first time. Both turn
   the guard's 401 (`isServerAccessChallenge`) into `reportAccessDenied()`,
   as the storage API client does with any 401, so a token changed on the
   server sends an open page back to sign-in; a late 401 for a token