fix(account): let session-key signer field mint a spare key

[montycheese]

Summary

• On a typical account (single owner, or after adding a second owner), every signer in the local wallet is already an owner or session key of that account, so every entry in the "Register a Session Key" Signer dropdown is disabled. Radix blocks clicks on disabled options entirely, so skSignerId never gets set and "Sign Authorization" stays permanently grayed out — session keys can't be created and no signer can be selected.
• Add a "+ New signer" action next to the Signer field that mints a fresh K1 key via the existing createSigner engine method and selects it immediately, giving the form a real, selectable signer without leaving the modal.

Test plan

• npx tsc --noEmit passes
• Reproduced the bug against a fresh single-owner account and confirmed clicking the disabled owner entry does nothing (Sign Authorization stays disabled)
• Verified the fix end-to-end in a browser: clicking "+ New signer" mints and selects a key, "Sign Authorization" enables, and the session key registers successfully

[cb-heimdall]

🟡 Heimdall Review Status

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Project	Deployment	Actions	Updated (UTC)
omni-ui	Ready	Preview	Aug 28, 2026 5:18pm