Identity default flag: clear-then-set pair has no transaction and no unique constraint

Summary

The "exactly one default identity" invariant is maintained by a clear-then-set pair of statements with no transaction, no row lock and no database constraint backing it. Two concurrent set_default() calls can leave a user with two identities flagged standard = 1.

Evidence

Measured on current master, PHP 8.5.10. First, the database accepts the state:

[CDFG-04] identities flagged standard for one user: 2
[CDFG-04] database rejected the second default: NO
[CDFG-04] unique index covering (user_id, standard): NO

There is no unique index in the shipped schema that would prevent it, so the invariant rests entirely on application code. And that code has no serialisation:

[CDFG-04] rcube_user mentions startTransaction   : NO
[CDFG-04] rcube_user mentions endTransaction     : NO
[CDFG-04] rcube_user mentions FOR UPDATE         : NO

The interleaving that produces it is the ordinary one: request A clears the existing default, request B clears (already cleared) and sets its own, request A sets its own — both end up with standard = 1.

Impact

Low to medium, data integrity only, no security impact. Consequences are user-visible rather than dangerous: which identity is picked as the default becomes dependent on row order, so compose may pre-select an unexpected sender. The mirror case — a moment where no identity is flagged default — is also reachable in the window between the clear and the set.

The trigger is mundane: a double-click on "set as default", or the same account being edited from two tabs.

Suggested fix

Two options, ideally both:

1. Wrap the clear-and-set pair in a transaction, using the existing rcube_db transaction helpers.
2. Add a partial unique index so the database enforces the invariant as well — on the databases that support it, e.g. CREATE UNIQUE INDEX ... ON identities(user_id) WHERE standard = 1 AND del <> 1.

Point 2 also protects against any future code path that forgets the transaction.

Happy to prepare a PR if that's useful.