feat(tui): add header visibility toggle

[hiromesh]

Add option to toggle session header visibility in TUI. Fixes #7799

The header shows session title, token usage, and cost at the top of the session view. Users can now hide it via command panel (ctrl+p).

Changes:

• Added header_visible config stored in KV
• Added "Hide header/Show header" command in Session category

How to test:

1. Run opencode and start a session
2. Press ctrl+p to open command palette
3. Search "header" or browse to Session category
4. Select "Hide header" to toggle visibility