feat: merge install.sh onboarding into atuin setup

[ellie]

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[greptile-apps]

Greptile Summary

This PR merges the install.sh onboarding flow into atuin setup, adding two new steps before the existing AI/Daemon config prompts: (1) offer to import shell history if fewer than 10 commands exist, and (2) offer Hub sign-up if not already logged in. The client.rs change correctly moves Setup to an early-return path so it no longer requires the DB/store to be initialized.

• The Hub auth flow (hub_signup) correctly uses the existing HubAuthSession poll-to-completion API with the 10-minute timeout — errors are caught and surfaced gracefully.
• Minor: a stray blank line is left in the match self block in client.rs after removing the old Self::Setup arm.

Important Files Changed

Filename	Overview
crates/atuin/src/command/client/setup.rs	Adds history import and Hub sign-up steps to the setup flow; logic is correct but prompt phrasing is awkward for the two new steps.
crates/atuin/src/command/client.rs	Moves Self::Setup to early-return before DB init and adds it to the unreachable!() arm; leaves a stray blank line in the match block (cosmetic).

_{Last reviewed commit: 4ba1769}