[FEATURE]: Add header visibility toggle

[hiromesh]

Feature hasn't been suggested before.

• 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.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Add configuration option to always hide the header #7536: [FEATURE]: Add configuration option to always hide the header

Feel free to ignore if this doesn't address your specific case.

[hiromesh]

#7798