handle donation for custom soroban

[koder-ns]

closes #55

Description

The donation flow currently only supports native XLM payments. This PR extends the system to accept custom Soroban tokens (e.g., USDC, EURC, or any other Stellar token contract), enabling campaigns to specify which asset they accept and allowing users to donate in their preferred token.

📁Files Changed

app/contract/contracts/src/donation.rs          - Added custom token support
app/contract/contracts/src/error.rs             - Added AssetMismatch error
app/backend/src/donations/donation.service.ts   - Asset validation logic
app/backend/src/donations/donation.types.ts     - New types/interfaces
app/backend/src/services/transaction-builder.service.ts - Approval + donation flow
app/backend/src/campaigns/campaigns.service.ts  - Accepted asset field
app/backend/supabase/migrations/*campaigns.sql  - Schema update
app/backend/src/__tests__/donations.e2e.test.ts - Integration tests
docs/CUSTOM_TOKEN_DONATIONS.md                  - Documentation (NEW)

[ayshadogo]

LGTM