Dont share accounts + account nicknames across networks

theres a couple of phases here

phase 1
- metamask accounts should be created + nicknamed per network, but addresses are still the same across networks. this is mostly a non-breaking change, because you can recover the same accounts.

phase 2
- metamask accounts should use a different HD path per network (this is a more correct following of the spec). this is a breaking change for non-ethereum mainnet uses

The issue names no files, tests, or entry points; start by locating account creation, nickname storage, and HD-path handling in the TypeScript extension. Compare current behavior across networks, then define and test phase 1's per-network nicknames with shared addresses before assessing phase 2's network-specific HD paths and its breaking-change implications.

Written by the indexing model from the issue text.