Sign-in markers: a tab's write erases sign-ins other tabs added, so app sign-out can skip their logout #1968

What happens

Each tab keeps a map of the entities it holds a remembered sign-in for, in localStorage. flagKeyAsSignedIn and flagKeyAsSignedOut (src/features/auth/store/authStore.ts) write that map back whole, from the copy this tab loaded at startup. A sign-in another tab added after that is erased by this tab's next sign-in or sign-out of anything.

The erased marker is what app sign-out uses to know which instances to log out of. So after it's lost:

• an app sign-out from either tab skips that instance's logout, and its server-side session outlives the sign-out;
• a removal or terminate that reads the markers (fix(cluster): log a removed self-hosted cluster's sign-ins out of its servers, then forget them #1966) can miss it too.

It takes two tabs, with a sign-in in one and any sign-in or sign-out in the other before a reload.

Related, from the same review of #1966: terminating a Fabric cluster (terminateCluster.ts) forgets this tab's sign-ins but not markers another tab added, so those outlive the forget. The same fix should cover that path.

Fix direction

• Read the current map from storage at each write, change one key, and write it back, instead of writing a startup copy.
• Have app sign-out log out every marker it finds in storage, not only the ones this tab holds in memory.
• Keep the two-tabs-write-at-once race in mind; a write per key, or a lock, closes it.

Review-Deferral: be82d5c881a51a56 severity=minor category=security
Review-Deferral: afad652932df433c severity=nit category=reliability

Raised by the cross-model review of #1966 (#1931). Pre-existing; not introduced there.

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