Voting period start banner

[bobo-k2]

Pull Request Summary

Voting period start banner will show 15 eras before Vote sub period start

Check list

• [ ] contains breaking changes
• [x] adds new feature
• [ ] 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 94b86bd):

https://astar-apps--pr1299-feat-dapp-staking-ba-0xqvef9w.web.app

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