Implement L2 token autodetection in the controllers

Currently, both mobile and extension only support mainnet token detection.

As a user, I should be able to get my tokens automatically detected in the most used L2s.

Start by locating the controller code responsible for the existing mainnet token detection and trace how mobile and extension detection currently work. Identify the most-used L2s in scope and any existing tests or entry points for token detection. Done means tokens are automatically detected across the agreed L2 networks in both supported contexts, with coverage for the relevant behavior.

Written by the indexing model from the issue text.