feat: add Stellar RPC retry handling

[kaycke1337]

Summary

• Add a shared Stellar retry helper with exponential backoff, ±25% jitter, retry telemetry, AbortSignal support, and StellarRetryExhaustedError
• Wrap Stellar Horizon/Soroban fetches and Soroban SDK calls in retry handling
• Surface exhausted network retries as "Network unstable — try again." with retry buttons in Stellar send/receive flows
• Add Vitest unit coverage for transient HTTP retries, non-retryable 4xx behavior, JSON parse retries, exhausted retries, and aborted signals

Closes #2

Validation

• npx prettier --check .
• npx vitest run
• npx tsc --noEmit
• npx vite build
• npx pnpm@10 test
• npx pnpm@10 build
• git diff --check

Note: Vite build emits existing dependency chunk/PURE annotation warnings, but exits successfully.

[vercel]

@kaycke1337 is attempting to deploy a commit to the truthixify's projects Team on Vercel.

A member of the Team first needs to authorize it.

[truthixify]

Heads up — we've just set up a develop branch as the integration target for all Drips × Stellar Wave contributions, and I've retargeted this PR's base from main to develop on your behalf. The diff is unchanged; nothing to do on your end.

Going forward, please branch from and target develop. We'll periodically merge develop → main as we cut releases. Thanks for contributing!

[truthixify]

Retry primitive (src/lib/stellar/retry.ts + tests) is exactly what issue #2 asked for. Conflict appeared after #13 (input validation) just merged, both touch StellarSend.tsx.

Rebase:

git fetch origin
git rebase origin/develop
git push --force-with-lease

When resolving StellarSend.tsx, keep #13's validation constants and balance-debounce logic, then wrap your retry hooks around the network calls. Thanks @kaycke1337.