fix: wire contract addresses and env vars from NEXT_PUBLIC_* with testnet defaults (#9)

[Chummy-debug]

Overview

Wires all 5 Soroban contract addresses, the RPC URL, and the network passphrase from process.env.NEXT_PUBLIC_* with sensible testnet defaults, and updates .env.example to document every environment variable consumed by constants.ts.

Related Issue

Closes #9

Changes

🔧 Contract address wiring — frontend/src/lib/constants.ts

• [EXISTING] All 5 contract IDs (REGISTRY, BOT_NFT, ACCRUAL, MARKETPLACE, TOKEN) already read from NEXT_PUBLIC_* env vars
• [EXISTING] SOROBAN_RPC_URL defaults to https://soroban-testnet.stellar.org
• [EXISTING] NETWORK_PASSPHRASE defaults to "Test SDF Network ; September 2015"
• [EXISTING] Individual exports + CONTRACT_ADDRESSES aggregate object

📄 Environment documentation — frontend/.env.example

• [FIX] Changed SOROBAN_RPC_URL example from placeholder https://your-soroban-rpc.example.com to actual testnet default https://soroban-testnet.stellar.org
• [ADD] Missing env vars documented: NEXT_PUBLIC_HORIZON_URL, NEXT_PUBLIC_TX_TIMEOUT, NEXT_PUBLIC_BASE_FEE, NEXT_PUBLIC_POINTS_PER_AMT, NEXT_PUBLIC_LEADERBOARD_LIMIT, NEXT_PUBLIC_POLL_INTERVAL_MS, NEXT_PUBLIC_COUNTER_TICK_MS

Verification Results

```

.env.example now covers every NEXT_PUBLIC_* var consumed by constants.ts

$ grep -oP "NEXT_PUBLIC_\w+" frontend/src/lib/constants.ts | sort > /tmp/code_vars
$ grep -oP "NEXT_PUBLIC_\w+" frontend/.env.example | sort > /tmp/env_vars
$ diff /tmp/code_vars /tmp/env_vars
✅ Complete parity — no missing vars
```

Acceptance Criteria	Status
5 contract addresses read from env vars	✅
RPC URL read from env var with testnet default	✅
Network passphrase read from env var with testnet default	✅
.env.example documents all NEXT_PUBLIC_* vars	✅

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