feat(webgw): per-user wallet sessions + invite register

Add the gateway backend for the wallet onboarding flow so each browser
session carries its OWN bus identity instead of sharing the single
operator client.

- POST /api/session (session.go): the browser hands its full wallet
  keypair (unlocked from the local encrypted key, over TLS) and the
  gateway spins up a dedicated bus client that acts AS that user. The
  private key lives only in process memory for the life of the session
  and is dropped on logout/shutdown. identityFromHex enforces the exact
  key sizes (sign_pub 32, sign_priv 64, kex_pub 32, kex_priv 32) that
  match cs.Identity on the Go side.
- POST /api/register (register.go): unauthenticated onboarding gated by
  a one-shot invite token. Validates the two PUBLIC key halves, then
  either consumes a configured --mock-tokens invite (local testing) or
  proxies to the bus POST /register (--register-url, bus >= 0.12.0). The
  handle/role come from the invite, never from the client.
- server.go: sessions move from a token->time map to a sessionStore of
  per-user *session records; auth() now resolves the session and passes
  its gateway to each handler. The legacy operator passphrase login
  (POST /api/login) is kept, bound to the shared operator gateway.
- main.go: build a busTemplate config that wallet sessions clone with
  their own Identity; wire --register-url / --mock-tokens.
- webgw_test.go: identity-size validation, hex-key validation, mock
  token parsing, and single-use register (201 then 409) using a fixed
  browser-derived wallet vector.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -12,5 +12,9 @@ worker.id
 /membershipd
 /worker
 /chat
+/webgw
 *.exe
 registry.db
+
+# Local session infra (machine-specific absolute paths; never distributed).
+.mcp.json