Add JWT Decoder tool

[GhDj]

Summary

• Add new JWT Decoder tool for inspecting JSON Web Tokens
• Client-side decoding - no data sent to server
• Color-coded token structure visualization
• Expiration status with human-readable time differences

Features

• Token Decoding: Decode header and payload from any JWT
• Visual Structure: Color-coded display (header=red, payload=purple, signature=cyan)
• Expiration Status: Shows if token is valid or expired with time remaining/elapsed
• Timestamp Formatting: Converts Unix timestamps to human-readable dates
• Registered Claims: Shows standard JWT claims (iss, sub, aud, exp, iat, nbf, jti)
• Sample Token: Load a sample JWT for testing
• Copy Functionality: Copy header or payload JSON

Test plan

• Verify JWT tool page loads at /tools/jwt
• Test sample JWT loading
• Verify header and payload decode correctly
• Test expiration status display
• All 273 tests pass

[GhDj]

Merged via cherry-pick into develop