fix: auto-retry mutation after SIWE re-auth on 401

[OlaGreat]

When a mutation (role assignment, policy update) fails with a 401 AuthError, the original request is now automatically retried once after the user successfully re-authenticates via the inline banner.

Changes:

• lib/wallet/siwe-context.ts: add PendingRetryCallback type and registerPendingRetry to SiweAuthContextType
• lib/wallet/providers.tsx: implement pendingRetriesRef queue; drain it in both signIn() and performSilentRefresh() after a successful session recovery; add registerPendingRetry to context value and SiweAuthContextValue interface
• admin/members/page.tsx: use registerPendingRetry in assignRole and removeRole onError handlers; remove React Query built-in retry for 401s to avoid racing with the re-auth banner flow
• admin/policies/page.tsx: use registerPendingRetry in policy mutation onError; add useToasts/ToastViewport for retry feedback
• test mocks: add registerPendingRetry: () => {} to all three useSiweAuth mock objects

Acceptance criteria met:

• 401 during mutation triggers inline re-auth banner
• Successful re-auth automatically retries original mutation once
• Second 401 on retry calls onRetryFailure (no infinite loop)
• Success/failure toasts shown for the retried mutation

Closes #327