Add Emergency Pause Mechanism

[Mkalbani]

Description

There is currently no way to halt the contract in case of an exploit or vulnerability discovery.

Expected Behavior

The contract owner should be able to pause all quest creation and completions instantly.

Proposed Changes

• Integrate OpenZeppelin Pausable
• Add whenNotPaused modifier to createQuest() and completeQuest()

Acceptance Criteria

• Owner can pause and unpause the contract
• All state-changing functions respect the pause state
• Tests cover paused and unpaused scenarios

[Lynndabel]

@Lynndabel has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello maintainer, i have gone through the task description and will like to work on this please. ETA: 6hours

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Lynndabel to this issue.