feat: add contract and horizon validation

[abore9769]

Overview

Validate contract addresses and verify configured Horizon endpoints are reachable before deployment.

Changes

• Contract Address Validation: Validates Soroban contract addresses (56-char, 'C' prefix, base32 charset)
• Horizon Connectivity Checks: Tests Horizon and Soroban RPC endpoints with configurable timeouts
• Error Classification: Distinguishes validation errors, transient errors, and configuration errors
• Async Endpoint Validation: Pre-deployment verification with actionable guidance

Implementation

Files Created

• apps/web/src/lib/stellar/contract-validation.ts
• apps/web/src/lib/stellar/contract-validation.test.ts (268 lines)
• apps/web/src/lib/stellar/endpoint-connectivity.ts
• apps/web/src/lib/stellar/endpoint-connectivity.test.ts (391 lines)
• apps/web/src/lib/customization/validate-endpoints.test.ts (279 lines)

Files Modified

• apps/web/src/lib/customization/validate.ts (+94 lines)
• apps/web/src/lib/customization/validate.test.ts (+64 lines)

Error Types

• VALIDATION: Invalid URL format (don't retry)
• TRANSIENT: 408, 429, 500, 502, 503, 504 (retry)
• CONFIGURATION: 404, 401, 403 (fix config)

Test Coverage

✅ 938 lines of tests (unit, integration, property-based)
✅ Contract validation (format, length, prefix, charset)
✅ Endpoint checks (Horizon, Soroban RPC, combined)
✅ Mock HTTP responses (various status codes)
✅ Timeout and error scenarios
✅ Edge cases and error differentiation

Usage

const result = validateCustomizationConfig(config);
const endpoints = await validateStellarEndpoints(config);

Resolves StellerCraft#52