When switching accounts, the target client connects with the token at
the account's stored cookieSlot. If that slot→token mapping is ever
wrong — e.g. corrupted client state persisted by an older build, or any
future slot desync — the connection succeeds as a *different* account
and the UI silently shows the wrong mailbox.
Add a post-connect identity guard: derive the connected session's
accountId (primary-identity email for OAuth, else the JMAP session
username) and compare it to the account being switched to. On mismatch,
drop the poisoned slot cookies and force a clean re-auth instead of
binding the wrong session.
This is belt-and-suspenders on top of 8b164c5, which fixed the slot
allocation that caused such a desync: that prevents new corruption,
this catches any residual/leftover mapping at switch time.
Adds a unit test for the canonicalisation (email vs JMAP username), the
make-or-break detail that avoids OAuth false-positives.
