fix: structured error codes across all contracts, contract ID validation, event indexer fields

[0xsteins]

closes #396 - Replace all string panic!() calls in invoice, pool, and credit_score contracts
with typed #[contracterror] enum variants; add 23 new InvoiceError codes (19–41), 3 new
PoolError codes (44–46), and 9 new CreditScoreError codes (1–9). All #[should_panic] tests
converted to try_* method assertions against specific error variants.

closes #402 - Add token address field to deposit and withdraw events in the pool contract
so off-chain indexers can identify which token was moved without reading contract storage.

closes #380 - Add module-load-time contract ID validation in frontend/lib/contracts.ts;
throws ConfigError with a descriptive message if any contract ID is missing or fails the
Stellar address format check (56 chars, starts with C). Guard is browser-only to preserve
CI/SSR compatibility, consistent with the existing assertEnvValid pattern.

closes #394 - Dashboard invoice list pagination was already fully implemented (PAGE_SIZE=20,
URL-reflected ?page=N, Prev/Next controls, "Showing X–Y of Z" counter); no changes needed.

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