Private Key Access

**Describe the usability problem**
_A clear and concise description of the specific usability issue_

Other than the basic "Account 1", accounts are apparently not imported to the desktop browser extension from the mobile app. Nor does the mobile app make the private key visible so that it can be manually typed into the browser extension to link the accounts across devices.

**Expected behavior**
_If possible, a clear and concise description / resolution of what you expected to happen_

The first, most obvious expected behavior is to have Metamask pickup _all_ accounts attached to a seed phrase automatically. Making it possible to see the private keys is also important, for obvious reasons.

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**User impact | Usage frequency | Feasibility scores**
_How important is this to the user, what the user wants to accomplish | Daily, weekly, monthly | Small, medium, large_

Start by reproducing account import and linking between the mobile app and desktop browser extension. Define whether completion requires importing all accounts attached to a seed phrase, exposing private keys for manual linking, or both, then verify the chosen behavior across devices.

Written by the indexing model from the issue text.