Manually Construct Transactions with Nested Proxy Accounts

[asiniscalchi]

As a Polkadot.js user,
I want to manually construct and send transactions from nested proxy accounts,
so that I can work around the current issue with signing transactions.

Acceptance Criteria:

1. Given: A nested proxy account structure (proxy_1 controlled by proxy_0)
   When: I manually construct a transaction using the extrinsics tab
   Then: The transaction should be successfully signed and sent without errors.

2. Given: A browser environment (Chrome/Firefox)
   When: Using Polkadot.js
   Then: I should be able to manually construct transactions from nested proxy accounts.

Current Behavior:
When trying to send tokens from a nested proxy account (proxy_1 controlled by proxy_0), the application displays an error: "Cannot sign with a locked key pair."

Expected Behavior:
Manually constructed transactions using the extrinsics tab should sign and send successfully.

Motivation for Change:
This workaround allows users to utilize nested proxy accounts for transactions despite the current signing issue.

Steps to Reproduce:

1. Create a pure proxy account controlled by Alice (proxy_0).
2. Create another pure proxy account controlled by proxy_0 (proxy_1).
3. Use the extrinsics tab to manually construct a transaction from proxy_1.
4. Attempt to send the manually constructed transaction.
5. Observe if the transaction is successfully signed and sent.

[asiniscalchi]

polkadot-js/apps#10615

[ccubu]

polkadot-js/apps#10615 (comment)