[FEATURE] customizable terminal input box cursor shape

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The input box cursor in Claude Code's TUI is displayed as a Block cursor by default. This differs from the standard Vertical Bar
cursor used in most terminal applications (zsh, bash, fish, etc.).

Current situation:

• The cursor shape is hard-coded — there is no configuration option to change it
• This is not related to Vim mode (which also shows a Block cursor)
• Terminal-level cursor settings do not affect Claude Code's TUI renderer
• The Block cursor feels unnatural for users accustomed to standard shell environments

Proposed Solution

Add a configuration option in settings.json to customize the input cursor shape:

```json
{
"cursorShape": "line" // "line" (default) | "block" | "underline"
}