Add Solana chain support (read-only balances + native SOL send)

[ndii-dev]

Summary

Globe Wallet is Stellar-only today. Follow-up to the EVM (Base + Ethereum) multi-chain slice — this tracks the same scope for Solana, which needs its own module since Solana uses a completely different account model and signature scheme (ed25519 + base58 addresses, no EVM-style checksums) from both Stellar and EVM chains.

Reference implementation

See the EVM module added in lib/evm/ (types in lib/types.ts under "EVM chains", lib/evm/networks.ts, lib/evm/evm.service.ts for read-only balance queries, lib/evm/evm-payment.service.ts for server-only signing, app/api/evm/balance + app/api/evm/send routes, components/app/evm-wallet-card.tsx). Solana should follow the same shape: a self-contained module alongside (not retrofitted into) the Stellar-specific IWalletService/AssetCode types, with mock-free read-only balance queries against a real testnet RPC and a server-only signing path gated behind an optional env var (so it fails closed with ERR_PAYMENT_NOT_CONFIGURED rather than fabricating a result, same as STELLAR_SOURCE_SECRET_KEY/EVM_SOURCE_PRIVATE_KEY).

Definition of done

• @solana/web3.js (or equivalent) added as a dependency
• Address validation (base58, 32-byte pubkey)
• Read-only SOL + SPL-token (USDC) balance queries against Solana devnet, verified against real on-chain data (not mocked) in the PR description
• Server-only native SOL send gated behind an optional SOLANA_SOURCE_PRIVATE_KEY env var, added to lib/env.mjs and .env.example
• API routes mirroring /api/evm/balance and /api/evm/send
• Unit + integration tests (mock the RPC client, not real network calls in CI)
• A minimal UI surface (balance card + send form), linked from somewhere reachable in the app