Feature request: configure rendered tab width in Codex CLI TUI

## Feature request

Allow users to configure the number of spaces used when rendering tab characters in the Codex CLI/TUI.

Rendered tabs currently appear to use a width of 8 spaces. In WSL, running:

tabs -2

before launching Codex does not appear to affect the rendering, so the CLI may be using its own tab-width handling rather than the terminal's tab stops.

A configuration option in config.toml—for example tui.tab_width = 2—would allow users to control the display width of tabs in rendered code, pasted text, and tool output.

The setting should affect rendering only; it should not modify the underlying content.
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