Add loading state to Sign in with Passkey button

[variablefate]

Problem

The "Sign in with Passkey" button takes a good second or two before the passkey card pops up. During that wait, the button gives no feedback, which can leave users wondering whether the tap registered.

Proposed fix

• Show a loading spinner on/next to the button while the passkey request is in flight.
• Disable / gray out the button while the request is in progress so users can't double-tap or assume nothing happened.
• Restore the button to its normal state once the system passkey sheet appears (or the request errors/cancels).

Acceptance

• Tapping the button immediately reflects a "working" state.
• Button is non-interactive during the in-flight request.
• State resets cleanly on success, cancel, and error paths.