Fix AppLayout tests to attach the mail store successfully

Problem

tests/unit/components/app-layout.test.ts uses a Repository mock that does not implement listAccounts. The mail store calls that method during attachment, so the suite repeatedly logs:

[mail-store] refreshFolders failed TypeError: repo.listAccounts is not a function

The tests still pass because the error is caught, meaning layout assertions run with folder attachment already broken. This can hide real regressions in account/folder-dependent layout behavior.

Acceptance criteria

• The AppLayout Repository mock implements the complete account/folder contract used during store attachment, including listAccounts.
• Test setup provides a coherent primary account and folder rows.
• The suite emits no refreshFolders failed errors.
• At least one assertion proves the mail store attached and loaded its folder state rather than merely rendering after a swallowed failure.

Start in tests/unit/components/app-layout.test.ts and trace the mail store attachment path to see which account and folder methods the Repository mock must provide. Run the AppLayout test, then verify the mock supplies a primary account and folder rows, no refreshFolders failed errors are emitted, and an assertion confirms the folder state loaded.