feat(identity-wallet): add OAuth frontend screens and auth_ready listener (MM-149 phase 7, tasks 3-4)
- Task 3: Create AuthenticatingScreen.svelte component
- Auto-invokes startOAuthFlow() on mount
- Shows spinner + status text while authenticating
- Calls onresolved() on success, onfailed() on error
- Follows DIDCeremonyScreen pattern with Svelte 5 runes
- Task 4: Update +page.svelte with three new OAuth steps
- Add three step types: authenticating, authenticated, auth_failed
- Add authError state to track OAuth errors
- Add onMount listener for auth_ready event from Rust backend
(emitted when app restarts with stored tokens in Keychain)
- Add rendering blocks for all three steps:
- authenticating: delegates to AuthenticatingScreen component
- authenticated: shows success message (ready for use)
- auth_failed: shows error code + Try again/Start over buttons
- Update complete step with Continue button to transition to authenticating
- Add oauth-screen and oauth-actions CSS classes
Verifies MM-149.AC7.1-4, AC8.1-2 (all OAuth-related acceptance criteria).