GNS - Testnets not present in Select network, and when the toggle is on, they are unintuitively under `Custom` despite being preloaded from start

**Describe the bug**

This is a feature request, to optimize steps when a user wants to Select a network.
Now: whenever I click on the network dropdown, if I want to select a Testnet, it's not straigtforward to find:
- Tabs: on the Select network modal, I see these 2 tabs: `Popular` and `Custom` -- nothing here suggests where I can find a Test network
- Not present in Popular
- Not present in Custom (by default)

so at this point, the user doesn't even know that we have pre-loaded testnets in the wallet by default. The user will only realize this if they click on the right menu >> Networks >> there we see a toggle (off by default) which says `Show test networks`
If that's enabled, once you go back to step 1 and try to Select a network, if the user clicks on Custom (which is still not intuitive for representing Testnets, as they are not custom, rather preloaded, despite being testnets), then they'll see the Testnet list.

**Expected behavior**

Make it more intuitive for a user to Select a Testnet: A couple of suggestions - not necessarily the good ones
- Display the Show testnets toggle, in the Select Network modal too, for the user to be able to see/unsee testnets in the modal itsel, instead of closing it and having to go to Networks Settings and perform the flow allover
- Placing them under Custom seems still unintuitive, as they are not Custom, but unsure how this could be solved 🤔

**Screenshots/Recordings**

https://github.com/user-attachments/assets/3ea2fc60-defb-4c9e-8f20-a7d8cc49d556

**Steps to reproduce**

1. Click on the network selector
2. See no Testnets in `Popular` tab
3. Click on `Custom`
4. See no Testnets in `Custom` tab
5. Close
6. Click on the right menu >>> NEtworks
7. Enable Testnets
8. Close modal
9. Click on the network selector again
10. See no Testnets in `Popular` tab
11. Click on `Custom`
12. See Testnets there ---> which doesn't make complete sense as they are not Custom networks, rather preloaded networks

**Error messages or log output**

```shell

```

**Detection stage**

On a feature branch

**Version**

13.2.0

**Build type**

None

**Browser**

Chrome

**Operating system**

Linux

Start by reproducing the network selector flow in Chrome, then compare it with the Networks settings flow that controls “Show test networks.” Review the open issue’s proposed alternatives and define a clear placement and labeling for testnets, with the selector behavior matching the chosen design.

Written by the indexing model from the issue text.