Implement Timer Module

[Mkalbani]

Description

Build a timer module that tracks time spent per puzzle for scoring and leaderboard purposes.

Acceptance Criteria

• A timer module exists under src/timer/mod.rs
• Timer can start, pause, resume, and stop
• Elapsed time is returned in milliseconds
• Timer integrates with the score module for time-based bonuses
• Unit tests cover all timer state transitions

[ayomideadeniran]

I'm a full-stack developer and I'd like to take this issue.
I understand the requirements and will follow the existing project structure, keep the implementation clean, and include proper tests where applicable.

ETA: 1–3 days (depending on scope)
Portfolio: https://knightsdev-portfolio.vercel.app/