[sup] ACM cookie-auth session: URL cluster/node context + temp-credential lifecycle

Part of #352. Heavily simplified by the cookie-auth model (was: #user login + pasted ACM key).

A dedicated ACM-mode ChCtx/session — no login form, no token, no key persistence.

Scope

• Read context from URL: cluster and node query params (supplied by the ACM console link). No cluster URL→id resolution.
• Cookie auth: all /api calls credentials:'same-origin', no auth header. A 401 = ACM session lost → redirect to ACM login (standard), not our concern to refresh.
• Identity: GET /api/account → engineer email, used as CH user= for query_log attribution. Model engineer-identity (from /account) separately from CH transport login (may be a returned login, else the email) and never assume they're the same.
• Temp CH credential lifecycle: mint via GET /api/cluster/{id}/support/credentials; read TTL from the response (don't hardcode); keep alive via /support/refresh or re-mint; single-flight so concurrent schema/dashboard requests don't mint in parallel. Handle both documented shapes ({password,ttl} / bare string / {login,password}).

Removed vs. original

• #user sentinel, ACM key in a password field, Chrome-password-store, no-localStorage/share-link/export rules for the key — there is no key in JS anymore.

Acceptance

• Loads cluster/node from URL; currentUser() is the engineer email.
• No credential stored in JS; cookie-only; 401 → ACM login redirect.
• Temp creds mint + refresh (single-flight), TTL from response, all response shapes handled.

Start by tracing the existing ChCtx/session flow and the ACM URL context, then inspect the /api/account and /api/cluster/{id}/support/credentials entry points. Verify the cookie-only requests, ACM redirect on 401, engineer-email attribution, and single-flight credential minting with every documented response shape; done means all listed acceptance checks pass.