[WIP] Fix unused DEFAULT_INDENT in progressbar.jl

[Copilot]

Task: Remove unused DEFAULT_INDENT constant and indent field

The issue reports that DEFAULT_INDENT is not being used in progressbar.jl. After analysis, this is confirmed:

• Analyze the codebase to confirm DEFAULT_INDENT is unused
• Verify that the indent field in MiniProgressBar struct is never referenced
• Run existing tests to ensure current state is working
• Remove the unused DEFAULT_INDENT constant from progressbar.jl
• Remove the unused indent field from MiniProgressBar struct
• Update README.md to remove indent parameter documentation
• Run tests to ensure changes don't break anything
• Commit the changes

Findings:

• DEFAULT_INDENT = 4 is defined but only used to set a default value for indent field
• The indent field is never actually used in any function (no p.indent references)
• README documents the indent parameter, but it has no effect
• Tests don't test indent functionality

Original prompt

On progressbar.jl, DEFAULT_INDENT是不是没用到

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