[frontend-contrib S11] Workers dogfood: zone panel + console route + island

[rickylabs]

Part of #922

Scope

plugins/workers ships the first real frontend: app.dashboard.panels queue-health zone component, /workers console route, QueueLive island, requires-declared procedures — the end-to-end proof a plugin install visibly changes the app with zero app edits.

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: extended scaffold.runtime — install -> route serves + nav + zone renders + island hydrates -> remove -> clean disappearance
• gate: defineFrontendTestSuite green for the plugin