fix: implement token expiration check in folksonomy API

[SaitejaKommi]

Description

The folksonomy API saves token timestamps to localStorage but never validates expiration. This causes:

• Using stale tokens that are no longer valid
• Unnecessary 401 errors before retrying with fresh token
• Silent authentication failures

Current Behavior

// Token saved with timestamp
saveTokenToStorage(token) // Saves token + Date.now()

// But timestamp is never checked!
getStoredToken() // Returns token even if it's 10 hours old ❌

Expected Behavior

// Token should be validated
getStoredToken() // Returns null if token is > 1 hour old ✅

Root Cause :

In src/api/folksonomy.ts:
Timestamp is saved but never used for validation
No expiration check in getStoredToken()

Solution :

Add expiration validation:
Define TOKEN_MAX_AGE_MS = 1 hour
Check token age in getStoredToken()
Return null and clear storage if expired

Security Impact :

Reduces token exposure time
Aligns with OAuth best practices
Prevents stale token usage