fix: implement token expiration check in folksonomy API

[SaitejaKommi]

Description

Implements token expiration validation to prevent using stale tokens in the folksonomy API.

Problem Solved

Previously, tokens were saved with timestamps but never validated for expiration, causing:

• Using stale tokens beyond their validity
• Unnecessary 401 errors from the API
• Silent authentication failures

Solution

Added expiration checking logic that:

1. Validates token age when retrieving from localStorage
2. Returns null if token is > 1 hour old
3. Automatically cleans up expired tokens
4. Triggers fresh authentication immediately

How Has This Been Tested?

• 3 new test cases added in src/api/folksonomy.test.ts
• Tests for expired tokens, fresh tokens, and cleanup
• All tests pass locally

Changes Made

1. src/api/folksonomy.ts

   • Added TOKEN_MAX_AGE_MS = 3600000 (1 hour)
   • Modified getStoredToken() to validate token age
   • Clears localStorage when token expires

2. src/api/folksonomy.test.ts

   • Added 3 new test cases for token expiration

Security Impact

✓ Reduces token exposure time
✓ Aligns with OAuth 2.0 best practices
✓ Requires fresh authentication every hour

Fixes #456

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