feat(frontend): add offline / RPC-unreachable error banner in dashboard

[libby-coder]

Problem

When soroban-testnet.stellar.org (or the mainnet RPC) is unreachable, getAllSchedules throws silently. The dashboard shows an empty state with no indication that it's a connectivity problem.

Fix

Catch network errors in the load() function and display a dismissable banner: "Could not reach the Stellar RPC — check your connection and refresh."

[euniceamoni]

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

Hi, I would love to work on this issue

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

[drips-wave]

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

🧑‍💻 @euniceamoni: 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 🌊