[FEATURE]: Add tui.json option to control content area max-width / horizontal padding

[alohaninja]

Description

After a recent upgrade, the horizontal margins/padding on the main content area in the TUI appear to have increased, making the readable content area noticeably narrower — especially on wide terminals. Tables and long-form output get compressed while there's unused space on either side.

This was previously raised in #838, which was closed without a user-facing config option being added.

Request

Add a tui.json option (e.g. content_max_width or content_padding) that lets users control how wide the main response content area renders. Something like:

// tui.json
{
  "content_max_width": 0, // 0 = full terminal width
  "content_padding": 1    // horizontal padding in columns
}

Currently tui.json supports scroll_speed, diff_style, mouse, etc. — but nothing for layout/spacing of the content area.

Current behavior

• Content area has fixed left/right margins that cannot be overridden
• The theme system only controls colors, fonts, and syntax highlighting — not layout
• No documented workaround exists for adjusting content width

Expected behavior

• Users can configure the content area width through tui.json
• Sensible default preserved, but overridable for users with wide terminals

Environment

• OS: macOS
• Terminal: wide-screen setup

[alohaninja]

Draft PR with a minimal implementation: #25146

Adds a content_padding option to tui.json (integer, 0–20, default 2) that controls the horizontal padding on the main session content area. Two files changed, backward-compatible — existing behavior is preserved unless the user opts in.

All CI checks are passing.

[alohaninja]

Updated PR with proper template (previous one was auto-closed for not following the PR template): #25171

All CI checks passing including compliance.