feat(auth): implement proactive SIWE session expiry warning banner (#226)

[TheDEV111]

Summary

Fixes #226

This PR introduces a proactive, dismissible warning banner for SIWE administrative sessions before the access token expires. Previously, session expiration only triggered an error banner after a failed mutation request (401), causing administrators to lose in-progress work (such as active edits in the Policy Editor).

Now, administrators are notified ahead of time with a live countdown and can extend their session with a single click before any requests fail.

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Changes Included

• Proactive Warning Helpers (lib/session.ts):
  • Defined DEFAULT_WARNING_THRESHOLD_SECONDS (120s / 2m).
  • Added getRemainingSessionSeconds(), isSessionExpiringSoon(), and formatTimeRemaining() helpers.
• Configurability (lib/config.ts):
  • Added warningThresholdSeconds to SiweConfig, populated via environment variables (NEXT_PUBLIC_SIWE_WARNING_MINUTES or NEXT_PUBLIC_SIWE_WARNING_SECONDS), defaulting to 2 minutes.
• Context & Provider Wiring (lib/wallet/siwe-context.ts & lib/wallet/providers.tsx):
  • Exposed isExpiring and warningThresholdSeconds through SiweAuthContext.
  • Unified SiweAuthContext instance between definition module and provider tree.
• UI & Accessibility (components/admin-guard.tsx):
  • Rendered a styled, accessible (role="status", aria-live="polite", aria-label="Extend your signed-in session") warning banner when isExpiring is active.
  • Included a live countdown timer ({currentSeconds}s), an Extend Session action button, and a Dismiss option.
  • Automatically clears the banner upon successful re-authentication.
  • Non-admin and unauthenticated users do not see the warning banner.
• Testing (test/siwe-proactive-warning.test.ts):
  • Added unit test suite validating warning helper functions, countdown logic, threshold detection, and AdminGuard render behavior.

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Verification

• Executed npm run test: 497 tests across 93 suites passed with 0 failures.
• Tested re-auth flow and dismissal interactions.