BUG: TUI fetchFoldersCmd silently drops per-account errors

[andrinoff]

Describe the bug

main.go:2511-2516:

go func(acc config.Account) {
    defer wg.Done()
    folders, err := fetcher.FetchFolders(&acc)
    if err != nil { return }
    ...
}(account)

Per-account errors are dropped on the floor. If an account's IMAP login is broken, the user sees the folder list silently miss those folders and never gets a notification.

Expected behavior

Collect per-account errors and surface them as a non-fatal warning in the TUI status line.