[Feature Request] Add persistent welcome banner setting to terminal UI

Bug Description

Requesting an opt-in setting to persist the boxed welcome banner in the terminal UI.

When launching Claude Code in a terminal, the welcome banner (logo + version + model + mode + cwd, drawn inside a Unicode border) renders for under a second before collapsing to a compact, unboxed header. I'd like a settings.json flag — e.g. "persistWelcomeBanner": true — to keep the boxed view pinned for the session.

Why this matters:
The boxed banner closely mirrors the visual polish of the VS Code extension's header. Losing it the moment the session loads makes the terminal experience feel downgraded compared to what's clearly already designed and shipped.

Why this should be a low-effort change:

• The boxed render path already exists and runs on every launch
• Opt-in via settings means default behavior is unchanged — zero risk to existing users
• No new UI, no backend, no model dependencies — pure client render logic
• Likely a one-line conditional gating the existing collapse trigger

Happy to test a flagged build.