[FEATURE]: focus mode to toggle off all the decorations

[ahkohd]

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

The user experience of opencode with large fonts isn’t great. This is partly because the decorations take up much more space than the actual content. Fortunately, you don’t need to see the decorations every time. A focus mode that only shows the input box and message area would be ideal, especially for users like me who use large fonts. This would free up valuable space.

fig: opencode on Ghostty terminal at 40px font size and 75% cell height.

In the proposed focus mode, only the message area and message box should be displayed, excluding other UI decorations like the title, status bar, all areas marked with red box in the screenshot should not be visible in focus mode.