fix: wait for ledger offscreen iframe load

[mikesposito]

Description

In order to instantiate a functioning communication between the offscreen iframe for Ledger and the LedgerKeyring (through LedgerOffscreenBridge), we need to make sure that the iframe is loaded before sending any message to it.

We currently wait for the offscreen page to load, but the iframe load is completely async and it will most likely be ready after the rest of the offscreen page, leaving messages proxied to the iframe hanging forever.

On a higher level, this is dangerous because everytime we try to send a message to the Ledger iframe the KeyringController controller-level mutex is locked, and any other operation will wait for its release to proceed - this creates a deadlock situation in the case the iframe does not respond to a message.

This PR makes Ledger iframe initialization into a Promise, and the offscreen page will wait for it to be resolved before sending the "ready" signal back to the extension. To avoid waiting forever, the Ledger initialisation promise races with a 5s timeout: in case of timeout, interactions with Ledger accounts will not work during the entire session (until the offscreen page is re-initialised, and another attempt is made to init Ledger).

Note that the UI setup is not affected by this change, since the Offscreen initialisation is awaited only when unlocking the wallet.

Related issues

Fixes:

Manual testing steps

N/A

Screenshots/Recordings

N/A

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Pre-merge reviewer checklist

• [ ] I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
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