[FEATURE]: Add header visibility toggle

[hiromesh]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

The session header (title, token usage, cost) always shows at the top of TUI. Users may want to hide it to save vertical space or reduce visual clutter.

Proposed Solution

Add a toggle option in the command palette (ctrl+p) to hide/show the session header.