docs: lead with library safety in the README and website

[hazeliscoding]

Rival prompt apps lost people's libraries in updates, so this puts Prompuff's safety up front. The next item from the roadmap's Getting found list.

• README: says the update check is the only thing Prompuff asks the internet, and adds that a library survives updates. Prompuff backs it up before an update changes it and once a day, and every later version opens it.
• Website hero: one sentence, "Updates back up your library before they change it."
• Roadmap: ticks the item. Its wording now matches the code, where a backup before an update is made only when the update changes the schema.