Add pause mechanism to Staking.sol (emergency circuit breaker)

[dDevAhmed]

• Fix: Explicitly handle rewardAmount == 0.
• Complexity: Low

[snakefood3232]

I see the need for an emergency stop to control the stake/unstake functions in case of a vulnerability. I'd handle this by inheriting OpenZeppelin's Pausable contract, implementing pause and unpause functions, and gating stake/unstake with whenNotPaused. Can have a PR ready in 2 days. I've built similar safety mechanisms in other DeFi contracts.