[S4] Add donate() function to API client

[Akshola00]

Description

Add the typed API function for donating to a pool.

Deliverables

• donate(poolId: number, amount: number, tokenAddress: string): Promise<{ unsignedXdr: string }>
• Calls POST /pools/:id/donate with JWT auth header
• Throws typed error on 400 (pool closed, invalid amount) or 401 (not authenticated)
• Exported from lib/api-client.ts
• npm run build passes

Notes

Depends on #58 (JWT interceptor). Requires Stage 3 POST /pools/:id/donate.

[DavisVT]

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

pls assign me this issue

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[drips-wave]

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

🧑‍💻 @DavisVT: 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.

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