Mb/2807 webauthn support for accounts

[mohnishbaker]

Add WebAuthn/Passkey authentication support

This PR adds passkey (WebAuthn) support as a second authentication factor and login method for Puter accounts.

What's changed:

Backend: New WebAuthnService handles credential registration and verification using @simplewebauthn/server. A new /api/v3/auth/webauthn router exposes endpoints for registration and authentication challenges/responses. A SQLite migration (0047) stores passkey credentials per user.

Frontend: New UIWindowWebAuthnSetup guides users through registering a passkey using @simplewebauthn/browser. The Security settings tab (UITabSecurity) now lists registered passkeys and allows removing them. The login window (UIWindowLogin) supports signing in with a passkey instead of a password.

How to test:

Log in to Puter and open Settings → Security
Click "Add Passkey" and complete the browser prompt
Log out and log back in using the "Sign in with Passkey" option

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