fix(auth): preserve saved values on re-authentication

Summary

Interactive glab auth login prompts for container_registry_domains, api_host, and ssh_host pre-filled from hostname-derived defaults, so a re-authentication (or a login after glab config set) silently overwrote the saved value when the user pressed Enter through the prompt.

This MR pre-fills each prompt from the value previously saved for the host, falling back to the derived default only when nothing is saved. The --api-host flag still wins over the saved value.

The pre-fill logic is extracted into initialAPIHostname, initialSSHHostname, and initialContainerRegistryDomains so it can be unit tested without driving the huh prompts.

Closes #8389 (closed).

Test plan

• go test ./internal/commands/auth/login/... passes (including new unit tests for the three helpers).
• golangci-lint run ./internal/commands/auth/login/... clean.
• lefthook run pre-push (build, lint, test-changed, generated-docs) passes.
• Manual verification of the reported scenario:
  1. glab auth login --hostname gitlab.com --web --git-protocol ssh.
  2. glab config set container_registry_domains "my.custom.registry" --host gitlab.com.
  3. Re-run glab auth login, confirm re-authentication, pick a sign-in method.
  4. The container-registry-domains prompt is now pre-filled with my.custom.registry instead of the derived default.

Follow-ups

The same pattern applies to git_protocol and api_protocol prompts (login.go:585-622). They aren't fixed here because both keys have Fallback: true in KeySchema, so cfg.Get can't distinguish "user set" from "schema default" without a new host-level accessor. Worth a separate MR.