server : (webui) let server send locally-defined default webui settings

[woof-dog]

There have been clashes, such as in #14041, with decisions about default llama.cpp client-side webui settings which are currently hardcoded and not adjustable by the llama-server host without modifying source code. Here I add a command-line argument for the default JSON-encoded client-side settings which are served by llama-server and read by the webui frontend only if no settings have been saved to local storage.

Notably this means that I won't have to maintain a fork and recompile the webui frontend code manually to disable particular settings or buggy functionality (such as setting pasteLongTextToFileLen to 0, helping #14041/#14251). Not all settings must be provided. For example,

defaultSettings.json:

{
  "pasteLongTextToFileLen": 0
}

can be passed via llama-server --default-client-config defaultSettings.json.

Ultimately this gives more control back to the user to customize the server's deployment.

(And please, if there are minor issues, such as if you want to change the name of the command line flag, reviewers feel free to make changes directly.)

[ngxson]

this is a bad UX practice because now user need to maintain a separated json file

it's better to re-design the config system and somehow re-use /props endpoint