feat(wallet): wallet-server HTTP API and web dapp

[saroupille]

Summary

• Adds `wallet-server`: an Axum HTTP server that exposes a private TzEL wallet over a local REST API (shield/transfer/unshield/scan/balance/address)
• Adds `web/`: a Vite + React dapp connecting to `wallet-server` in real mode, with a mock mode for UI development without a live node
• Adds bech32 encoding for `PaymentAddress`, derives `PartialEq`/`Eq` on it, and removes unused `verify_wots_signature_against_leaf`
• Adds `docs/wallet_server.md` with a step-by-step manual testing guide

Architecture

```
web dapp (React) → wallet-server (Axum) → tzel-operator (ledger API)
→ proving-service (optional STARK proofs)
```

Proof modes

• `--skip-proof`: skips STARK proofs, for demos and integration testing
• `--proving-service=`: delegates proof generation to a remote HTTP proving service

API endpoints

Method	Path	Description
`GET`	`/balance`	Returns `{"private_balance": N}`
`POST`	`/address`	Returns a PaymentAddress JSON (advances `addr_counter`, persists wallet)
`POST`	`/scan`	Pulls new notes from ledger, removes spent ones
`POST`	`/shield`	Shields L1 funds into the private pool
`POST`	`/transfer`	Transfers between shielded addresses
`POST`	`/unshield`	Withdraws to a public L1 address

Design decisions

• Wallet locking: state is held in `Arc<Mutex<(WalletFile, ...)>>` — all handlers hold the lock for their full duration (no partial updates)
• Save ordering: `save_wallet` is called only after a successful ledger POST, so a network failure does not consume an address slot or mark notes as spent
• `/address` is POST, not GET: because it mutates `addr_counter` and persists to disk
• 502 on ledger errors: all ledger-unreachable errors return `502 BAD_GATEWAY`, not `400`

Test plan

See docs/wallet_server.md for the full manual testing guide.

• Build: `cargo build -p tzel-wallet-app --bin wallet-server`
• Start server with `--skip-proof` against a running `tzel-operator`
• `GET /balance` returns `{"private_balance": 0}` on a fresh wallet
• `POST /address` returns a valid payment address JSON
• `POST /shield` with a funded sender succeeds and increments balance after scan
• `POST /scan` detects incoming notes and removes spent ones
• `POST /transfer` moves funds between two wallet-server instances
• `POST /unshield` withdraws to a public L1 address
• Web dapp: `cd web && npm install && npm run dev`, verify all operations via UI

🤖 Generated with Claude Code