Staker rewards calculation fix

[bobo-k2]

Pull Request Summary

I managed to reproduce and fixed staker reward calculation

My rewards before fix: 995.55 ASTR

My rewards after fix: 664.58 ASTR Claimed rewards

Check list

• [ ] contains breaking changes
• [ ] adds new feature
• [x] modifies existing feature (bug fix or improvements)
• [ ] relies on other tasks
• [ ] documentation changes
• [ ] tested on mobile devices

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 8617636):

https://astar-apps--pr1224-fix-staker-rewards-0ikm5b5w.web.app

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