Restore install-folder (--prefix) choice in the dash onboarding wizard

[michaelroy-amd]

Context

PR #67 retires the legacy tui.rs and routes rocm bootstrap setup to the dash onboarding wizard. The legacy interactive setup let users pick the TheRock/ROCm install location (surfaced on the CLI as --prefix PATH). The current dash onboarding wizard has no install-folder picker, so the guided bootstrap setup flow installs to the default location only.

This was deferred, not removed by design — tracked here so it isn't lost.

Scope

• Add an install-location step to the onboarding wizard (crates/rocm-dash-tui/src/ui/onboarding.rs), feeding the existing --prefix plumbing (install_manager.rs already threads --prefix, and the chat tool path still supports it).
• Decide the default and let the user override it, consistent with the pre-retirement behavior.

Notes

• The Release/Nightly channel toggle — dropped in the same retirement — was already restored separately in feat(dash): restore Release/Nightly channel toggle + version pin in onboarding #73, so this issue is scoped to the install-folder choice only.
• rocm bootstrap setup's non-interactive fallback message was updated in Retire legacy tui.rs: route rocm bootstrap setup to the dash onboarding, then delete the ~45k-line module #67 to stop advertising a folder picker; revert/adjust that wording once the picker returns.