Add unit tests for the FlowRatePreview/DurationPicker calculation logic

[Chuks-coderr]

Description

FlowRatePreview's per-second/hour/day/month math and DurationPicker's seconds conversion have no test coverage at all. Add focused unit tests for this calculation logic specifically (narrower scope than the full component suite in Issue 30).

Acceptance Criteria

• Feature is fully implemented
• Tests pass
• Lint/type checks pass
• PR references this issue number

Notes

⚠️ Do not begin coding until you have been assigned by the maintainer.
Branch as feat/N-short-title where N is this issue number.

[edehvictor]

@edehvictor has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Please can I work on this. I understand the scope of the issue

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

[drips-wave]

Congratulations, @edehvictor! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @edehvictor: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[Emmo00]

@Emmo00 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i would love to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Emmo00 to this issue.

[laragrey]

@laragrey has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hi, can i work on this?

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

[drips-wave]

Congratulations, @laragrey! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @laragrey: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊