arg: detect console width dynamically for CLI help wrapping

[tanishqtayade]

Overview:-

Make CLI help text wrapping depend on the actual console width instead of a
fixed 70-character limit, improving readability across different terminals and
addressing the existing TODO in 'common/arg.cpp'.

Additional information:-

Add a small 'get_console_width()' helper in 'common/arg.cpp':
On Windows, use 'GetConsoleScreenBufferInfo()' to get the console width.
On POSIX systems, use 'ioctl(TIOCGWINSZ)' to get 'ws_col'.
On Emscripten or when width detection fails, fall back to 70 characters to preserve existing behavior.
Use this width to compute 'n_char_per_line_help' in 'common_arg::to_string()'.
This addresses the existing TODO in 'common/arg.cpp':
'// TODO: detect this based on current console'.

Requirements:-
I have read and agree with the contributing guidelines.
AI usage disclosure: I used an AI assistant to help draft and refine this change and description, and I have reviewed, built, and tested the code myself.

[ngxson]

please respect the PR description template

[tanishqtayade]

@ngxson ,PR description is now updated to align to the template (Overview / Additional info / Requirements), and AI usage notification added.
The change is limited to 'common/arg.cpp', and I’ve tested on Windows with:

• '.\build\bin\Release\llama.exe cli --help'
• '.\build\bin\Release\test-arg-parser.exe'
  Please let me know if you need anything changed in the workflow or naming.