Fix null vs undefined in GitHub OAuth, use arctic's
createAuthorizationURLWithPKCE for OIDC, loosen api-utils type
signature to avoid SvelteKit route-specific generics, add @types/zxcvbn.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Add Google, GitHub, and generic OIDC provider modules using arctic v3.
Providers are auto-enabled when env vars are set. Google/GitHub use PKCE,
OIDC uses OpenID Connect discovery.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>