[frontend-contrib S21] AI frontend (durable chat route + assist launcher)

[rickylabs]

Part of #922

Scope

plugins/ai live chat: durable-session route + contributed specialized stream-proxy route (host principal authorization), Chat island from the productionized stub, AssistLauncher behind a capability check; starter chat page remains the eject path.

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

Acceptance

• gate: durable sessions survive reload/reconnect through the contributed route
• gate: defineFrontendTestSuite green