Feature - Subgraph Library

# Proposal and mockup
A dedicated side tab for subgraphs and accessing published subgraphs.

There is all of this space in this sidebar, I'm sure 1 more button could be added. With the 278 different nodepacks everyone has installed, no one really puts buttons in the sidebar. So it seems like a convenient blank space to put a subgraph library, that is easily accessible. And it would follow existing UI conventions without doing weird things that no one expects.

# Why?
Because searching is only good if you can remember the name of something
Fuzzy search doesnt fix your bad memory unfortunately

Because the node tab keeps resetting and it's annoying and clunky and slow to use - and you cant favourite the category to push it to the top where it is convenient to access

https://github.com/user-attachments/assets/173e6212-5c65-470c-8daa-e91d42dc4efd

# Huh?

I've seen some noobs completely stumped because they cant even find their published subgraphs.

One would expect subgraphs to be treated slightly differently or categorized slightly diferently from normal/custom nodes.
We are adopting the usage of subgraphs more and more. They're convenient to use, but not as convenient to access.