Simplify repository setup

The setup ask for the BorgBase token, but considering that in 99% we only need one, this could be advantageously saved in Bitwarden (`master`) account. The setup would then, just need the Bitwarden credentials which is simplier.

I propose that we agree on the name of the BorgBase item, like for example "BACKUP_BORGBASE_TOKEN". If the item is there then use it, otherwise ask the setuper to give it (interactivly).

Start by tracing the repository setup flow and how it currently requests the BorgBase token. Review the proposed Bitwarden item name, `BACKUP_BORGBASE_TOKEN`, and the fallback interactive prompt. Done means setup can reuse the stored token when present and still ask for it when absent.

Written by the indexing model from the issue text.