Fix token validation to check JWT expiry

[wistfulvariable]

Summary\n- Validates JWT structure (3 parts)\n- Checks expiry timestamp against current time\n- Returns false for empty/malformed tokens\n\nFixes #1\n\n## Test plan\n- [ ] Unit tests for expired tokens\n- [ ] Unit tests for malformed tokens\n- [ ] Integration test with real JWT

[wistfulvariable]

The JWT parsing looks good but we should add try/catch around the JSON.parse and atob calls — malformed base64 will throw.

[github-actions]

LGTM! The JWT expiry check looks correct. One suggestion: wrap the atob/JSON.parse in try/catch for malformed tokens.

[github-actions]

LGTM! The JWT expiry check looks correct. One suggestion: wrap the atob/JSON.parse in try/catch for malformed tokens.

[github-actions]

LGTM! The JWT expiry check looks correct. One suggestion: wrap the atob/JSON.parse in try/catch for malformed tokens.