feat: add default networks

[brusherru]

Added two default networks in the list:

• MainNet
• TestNet 13

This networks won't be stored in the localStorage, so they might be replaced easily in the future (TestNet). And it is still possible to add custom networks and switch between them.

The only possible bad edge case — if we remove testnet / add one more network — it may switch to this network automatically (because selected index is stored in the localstorage, while default networks are indexed all together).

But anyway, I believe it will make easier to check out the wallet by testers ;)

P.S. It is based on top of PR #23, so it includes all previous fixes.

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