[iOS] Add a customizable CLI-style header or status bar

[amanthanvi]

Summary

The app would benefit from a first-class customizable header or status bar with CLI-style information density.

Related work

This looks like follow-up feedback to #15, which improved the current header and status presentation.

Problem

The header is one of the best places to expose high-value session state on mobile, but a fixed layout cannot satisfy everyone. Different users care about different pieces of context, and right now the information density and readability feel constrained.

Requested behavior

• Let users choose which fields appear in the header or status bar.
• Candidate fields include model, connection target, project or cwd, branch, approval or sandbox state, plan mode, and usage.
• Support at least a compact mode and a more detailed mode.
• Keep the default clean, but allow power users to make it denser and more CLI-like.

Why this matters

For long-running sessions, small pieces of ambient context make a big difference. The current app already hints at this; making it customizable would push it much closer to the usability of the CLI.

Acceptance criteria

• Users can enable or disable key status items.
• The default configuration remains clean on phone-sized layouts.
• Important behavioral state such as plan mode, connection state, approval state, and usage can be surfaced without guesswork.
• Customization persists across launches.