fix(multisig): await creation submission before navigating away

[n13]

Summary

• Multisig creation previously fire-and-forgot the submit (unawaited), so the UI navigated Home before the network accepted the extrinsic — inconsistent with the normal transfer flow (which waits for the RPC hash, see wait for RPC hash before showing success screen #524).
• startMultisigCreation now awaits submitCreateMultisigExtrinsic and the returned extrinsic hash before completing. Only finalization polling stays in the background.
• On submit failure it removes the pending creation record and rethrows, so AddMultisigScreen surfaces the error inline via its existing error toaster and stays on screen instead of optimistically navigating. This replaces the asynchronous submitFailed toast with synchronous error handling at the call site.

Test plan

• Create a multisig with sufficient balance → screen waits until the tx hash returns, then navigates Home; pending creation continues tracking to finalization.
• Force a submit failure (e.g. offline) → error toaster shown, stays on Add Multisig screen, pending record cleared, button re-enabled.
• Insufficient balance / already-exists preflight → specific error toasters unchanged.
• flutter analyze clean (verified).

[n13]

not what I had in mind, closing