frontend: Remove "using namespace std" from headers

[gxalpha]

Description

This is very dangerous as it propagates down everywhere the headers are included in.
Imo we should avoid using namespace std in general, but in headers it should be an absolute no-go.

Motivation and Context

I was baffled how this code works without std:: in front of find_if:

obs-studio/frontend/utility/YoutubeApiWrappers.cpp

Lines 34 to 39 in 0b12296

	bool IsYouTubeService(const std::string &service)
	{
	auto it = find_if(youtubeServices.begin(), youtubeServices.end(),
	[&service](const Auth::Def &yt) { return service == yt.service; });
	return it != youtubeServices.end();
	}

So I searched the entire source code to find if (pun intended!) there is anywhere where using namespace std is in a header.
The two spots I did find are in this PR. They do not affect the code above. I am still baffled. (but at least fixed the ones I did find).

Please tell me if you know why the above code works, haha.

How Has This Been Tested?

macOS 26, still compiles.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

Good catch. Should probably explicitly fix that find_if separately.