feat: add /api/v1/auth/me endpoint for token freshness introspection (Closes #382)

[waterWang]

Summary

Adds a new GET /api/v1/auth/me endpoint that returns the authenticated user's Stellar address and token expiration time. The frontend getAccessToken() now consults this endpoint before requesting a new challenge, skipping the full re-auth flow when the existing token has more than 60 seconds of TTL remaining.

Backend Changes

• New GET /api/v1/auth/me endpoint in auth.controller.ts — protected by JwtAuthGuard, returns { address, expiresAt }
• Updated JwtStrategy.validate() to also return the exp claim from the JWT payload

Frontend Changes

• Updated getAccessToken() in lib/api.ts:
  • When a saved token exists and matches the public key, calls /api/v1/auth/me to check freshness
  • If >60s TTL remaining → returns the existing token (no re-auth)
  • If <60s TTL or network error → clears the token and re-authenticates

Tests

• Updated api.spec.ts to mock the /me endpoint and verify the new flow
• Updated api.spec.tsx to verify that the saved-token path calls /me for freshness

Acceptance Criteria

• Unit test for token freshness returns correctly (via /me endpoint)
• Frontend getAccessToken() skips re-auth when >60s TTL remains

Closes #382