web: move the SIWE challenge below the facade

[FSM1]

Goal

Move the SIWE nonce fetch below the facade so apps/web holds no seam of its own.

Problem

facade.siweLogin(message, signature) exists, but nothing exposes the challenge that an
EIP-4361 message must embed. The engine's API client already has siwe_challenge()
(crates/engine/src/api/client.rs) against POST /auth/siwe/challenge; it is unreachable
from the facade.

Landing #804 therefore left apps/web/src/auth/siweNonce.ts doing a plain fetch against
the API's public challenge endpoint. That is the only direct API call in apps/web and it
contradicts blueprint/web-client.md "apps/web ... no crypto, no seams, no tokens" — its
only vault-facing dependency should be packages/client.

The nonce is public, single-use, and carries no key material, so this is an architecture
debt, not a live vulnerability.

Work

• Add a siweChallenge read to the engine facade over the existing ApiClient::siwe_challenge,
  and bind it in crates/wasm.
• Thread it through packages/client: protocol.ts request/response, transport.ts,
  the correlated/broadcast transports, worker/serve.ts, and facade.ts.
• Delete apps/web/src/auth/siweNonce.ts and have WalletLoginButton read the nonce
  from the facade.

Note

siweLogin is a Command, so it is refused with NotStarted before facade.start. Settle
whether the wallet method belongs on the cold login page at all, or only as a linked
secondary method on an already-started engine, while doing this.

Gate

Client Browser Suite + web unit.

Depends on

• web: harvest the v1 login UI and rewire it to the facade #804 — this issue deletes apps/web/src/auth/siweNonce.ts and rewires the WalletLoginButton that web: harvest the v1 login UI and rewire it to the facade #804 landed.

Shares a threading surface with #914: both add a facade member and carry it through crates/wasm and
packages/client (protocol.ts, transport.ts, the correlated/broadcast transports, worker/serve.ts,
facade.ts). Land one, rebase the other.

Blocks

• web: give the wallet method a started-engine surface #938 — gives the wallet method a started-engine surface. This slice settled the placement question its "Note" raised and gated siweChallenge behind start; web: give the wallet method a started-engine surface #938 acts on that settlement and moves WalletLoginButton off the cold login page.
• web: give the Http seam a request deadline #939 — gives the web Http seam a request deadline. This slice deleted the AbortSignal.timeout(10_000) that apps/web/src/auth/siweNonce.ts carried, so web: give the Http seam a request deadline #939 restores that behaviour at the seam layer where it belongs.
• client: generalize the facade read rail #942 — generalizes the facade read rail. This slice added the third read, siweChallenge, which is what fires the rule of three; its threading is deliberately mechanical so client: generalize the facade read rail #942 can collapse it.

Part of #642

[FSM1]

Two more things to fold into this issue, found reviewing #911:

The facade's SIWE signature encoding does not match the API's DTO. crates/engine/src/facade.rs sends hex_lower(&signature) — unprefixed — while apps/api/src/auth/dto/auth.dto.ts requires /^0x[0-9a-fA-F]{130}$/. Every wallet login would 400 at DTO validation. The unit test at facade.rs asserts the unprefixed form, so it currently pins the bug, and the contract case passes only because it calls ApiClient::siwe_login directly rather than driving the facade command. Fix the encoder, correct the assertion, and add a contract case that goes through the facade.

SiweService.verifySiweMessage does not validate uri. It passes only domain and nonce to validateSiweMessage, so the uri field the client sets is unverified. The signed message also carries no expirationTime, leaving freshness entirely on the server-side nonce TTL.

[FSM1]

Settling the Note while landing this: the wallet method belongs only on an already-started
engine, not on the cold login page.

Command::SiweLogin is refused with NotStarted before facade.start, and it cannot be otherwise —
SIWE authenticates the account against the API and cannot produce the Core Kit login secret a vault
cold start needs. blueprint/engine.md "API client" and blueprint/web-client.md "Login and
identity" both call it a secondary method. So facade.siweChallenge is gated the same way: a nonce
the engine could never spend is refused before the host prompts a wallet for it rather than
after, which is strictly fewer wasted user actions for the same outcome.

The placement half could not land here — apps/web/src/App.tsx has only / and /files/:nodeId?,
so there is no started-engine surface to move WalletLoginButton to. Filed as:

• web: give the wallet method a started-engine surface #938 — give the wallet method a started-engine surface, move the button off LoginPage, and
  correct the blueprint/web-client.md route table that still lists / as hosting SIWE.

Also filed, from the seam this change now routes through:

• web: give the Http seam a request deadline #939 — give the Http seam a request deadline. The deleted apps/web/src/auth/siweNonce.ts
  carried its own AbortSignal.timeout(10_000) because it bypassed the seam; FetchHttp.send passes
  no signal, so every engine API call is unbounded today. Restoring that at the seam is the right
  layer, but it needs a per-request-class deadline rather than a flat one.