Mock server cannot produce a failed firing, so the error UI is unreachable in the demo

[BenSeverson]

Found while verifying #164.

The mock kiln (web_ui/mock-server/) has no way to represent a firing that failed:

• every history record is hardcoded errorCode: 0 (router.ts:78, :88, :98)
• the simulator never enters status: "error"
• /api/v1/system always reports lastErrorCode: 0 and emergencyStop: false

So none of the error UI added in #235 — the dashboard error banner, the history detail cause, the Settings "Last Error" description, the emergency-stop guidance — is reachable in npm run dev, in the Vitest suite, or in the published demo at https://benseverson.github.io/bisque/. Verifying that PR required stubbing window.fetch in the browser by hand.

This is the same firmware/mock divergence class as #131, which has now come up five times (idle broadcast, delayMinutes, profileId omitted, profileId cleared on stop, and this).

What would fix it

Give the mock a way to reach a failed state, so the error path is exercisable:

• a POST /api/v1/mock/fault (dev-only) or a scripted scenario that trips a fault mid-firing and leaves status: "error" with a non-zero lastErrorCode
• at least one seeded history record with outcome: "error" and a real errorCode
• emergencyStop: true reachable

The demo is the only way most people will ever see this UI, and right now it shows the happy path exclusively.