light-client: browser wallet + Ed25519 signed transfers

[liu971227-sys]

Implements bounty #62 (light client):

• Adds browser light client served from node origin: /light + /light-client/*
• Generates/validates BIP39 24-word seed (English wordlist vendored)
• Derives RTC address from Ed25519 pubkey: RTC + sha256(pubkey)[:40]
• Signs canonical JSON message (sort_keys + separators) and posts to /wallet/transfer/signed
• Adds DB_PATH env override + creates transfer_nonces table needed for replay protection

Local E2E test:

• Start node with RC_ADMIN_KEY set
• POST /wallet/transfer/signed returns 200 ok, phase=pending, verified=true

[Scottcjn]

Changes Requested — Promising concept, needs refinement

A browser-based light client with Ed25519 signed transfers is interesting, but:

1. Verify against live API: Has this been tested against https://50.28.86.131? Include screenshots or curl output
2. Key management concerns: How are private keys stored in the browser? LocalStorage is not secure for Ed25519 keys
3. Endpoint compatibility: The transfer endpoint format must match our server's /wallet/transfer/signed schema exactly (check field names: from_address, to_address, amount_rtc, nonce, signature, public_key)
4. No bundled dependencies: If using external libraries (tweetnacl, etc.), document how they're loaded

This could be a great contribution — please address these points and resubmit.