AccessSecretStorageDialog: fix inability to enter recovery key (#30090)

* BaseDialog: fix documentation, and make `onFinished` optional

Since `onFinished` isn't used if `hasCancel` is false, it's a bit silly to make
it mandatory.

* AccessSecretStorageDialog: fix inability to enter recovery key

Wrap AccessSecretStorageDialog in a `BaseDialog`. The main thing this achieves
is a `FocusLock`.

* playwright: factor out helper for verification

We have two copies of the same code, and we're about to add a third...

* playwright: test for verifying from Settings

* Add a unit test for BaseDialog