Add Fast mode status-line indicator

[etraut-openai]

Addresses feature request #13660

Adds new option to /statusline so the status line can display "fast on" or "fast off"

Summary

• introduce a FastMode status-line item so /statusline can render explicit Fast on/Fast off text for the service tier
• wire the item into the picker metadata and resolve its string from ChatWidget without adding any unrelated thread-name logic or storage changes
• ensure the refresh paths keep the cached footer in sync when the service tier (fast mode) changes

Testing

• Manually tested

Here's what it looks like when enabled:

[mrkn]

I personally like the "⚡️GPT-5.4" label shown in Codex.app. I’d be happy if Codex CLI also had a mode that could display it in the same way.

[joshka-oai]

This needs an insta snapshot test that asserts how this looks rather than asserting on values as it makes it easy to see visual drift better. Also +1 to the lightning bolt idea.