T3-010: Implement sign message flow for authentication

Track: 3 — Solana Phone / Web3
Section: Phase B: Wallet Integration via MWA (56h)
Status: [ ] Todo
Repo: app-user
Branch: feature/T3-010-wallet-sign-auth
Estimated Hours: 6h

Done When

App requests wallet signature on a nonce message. Backend verifies signature matches stored public key. Auth alternative to email/password

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