webui: Disable pasteLongTextToFileLen by default

[mashdragon]

This PR disables the pasteLongTextToFileLen feature by default in the hope that someone can design a better interface for using it.

Discussion: #14042

[ngxson]

People have been using it and no one complain except for you. I don't see why this should be disabled by default. This is the same behavior on claude/chatgpt and it make sense for most users.

[mashdragon]

The main reason (#14042) is that the defaults make it impossible to construct large prompts either inside the textarea or by pasting them in from outside, and we can make the UX better by using keyboard shortcuts instead of modifying the paste functionality based on an arbitrary character count.

Which gives me an idea you might be on board with: why don't we create a keyboard shortcut that disables the file paste? That way, I can still use the textarea as an editor or paste in prompts from an external editor, and it would only cost me an additional meta key: we can have Ctrl/Cmd+V/context paste continue to use the current logic and Ctrl/Cmd+Shift+V pastes directly. What do you think of that?

[woof-dog]

I also have a problem with this. I did not report it before because it's annoying but tolerable enough that I didn't log into github until now. The feature definitely needs UX improvement. Whenever I use a new browser or device I have to go into the settings and disable it. There needs to be some default settings configuration at the llama-server binary level to alleviate this contention