* fix(accounts): dismiss discovery explanations when the popup closes

The accounts panel kept the "no named accounts" or "cannot switch accounts"
explanation after an Accounts… click until the CLI or CodexBar configuration
changed. AccountsController now tracks the explanation separately from the
provider's availability; main.qml dismisses it when the popup closes, while
an unsupported provider's Accounts… entry stays disabled.

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

* fix(accounts): keep dismissed notices silent for late replies

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

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Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>