feat(auth): Continue with Nostr splash button + password manager autofill

[barrydeen]

Summary

• Replace the splash's separate Create Account and Log In buttons with a single Continue with Nostr button (purple-ostrich vector logo) alongside the existing Google button.
• Tapping it opens a ModalBottomSheet with an nsec/npub field, a Log In primary, and a Create new account secondary.
• New accounts: after key generation, NostrCredentialSaver calls CredentialManager.createCredential(CreatePasswordRequest) so the device's password manager prompts to save the nsec keyed by the npub.
• Existing accounts: on first focus of the nsec/npub field, NostrCredentialSaver.loadSavedNsec runs a GetCredentialRequest(GetPasswordOption) so the system password-manager picker offers saved keys; the chosen password fills the field.

Test plan

• Cold start with no account → splash shows Continue with Google and Continue with Nostr; old two-button layout is gone.
• Tap Continue with Nostr → bottom sheet opens with the ostrich, title, nsec/npub field, Log In, divider, Create new account.
• Create new account → password-manager save sheet appears with the new npub as the username; accepting saves the nsec, cancelling still completes onboarding.
• Reopen the sheet, tap the nsec field → password-manager picker appears and chosen key autofills the field; tapping Log In logs in.
• Pasting a fresh nsec/npub manually still works; pasting an invalid string surfaces the existing error text.
• Add-account flow (drawer → Add account) routes through the same splash + sheet and lands in EXISTING_USER_ONBOARDING / LOADING correctly.