Add hierarchy feature for tabs

[firdausai]

horse.browser have this tab hierarchy where new tabs open under the parent tab you clicked from, and I think we should have one in Zen too.

I'm constantly opening new tabs (probably have like 20+ open right now) and completely lose track of where I found stuff. It's hard to keep track when I'm researching something.

Idea:

• Click link in Tab A → new page opens as a "child" under Tab A
• Maybe show some visual connection between related tabs
• Option to collapse tab groups when you're done with them

This would be nice for keeping track of related stuff. Like when I'm shopping and open 5 different product pages from search results, or checking documentation while coding, or in google scholar and open up a lot of the research article that comes from the search result. Right now all of the tabs tabs just blend together in one long row. The hierarchy would let me actually see which tabs belong together.

Here's a visual from their website: