fix(web): refresh ReAct task state

[wolfkill]

Summary

• replace the ReAct sidebar New Task link with an explicit reset action so it works even when already on /
• notify the sidebar after a ReAct conversation stream completes and refresh the task list, including a delayed retry for backend persistence
• centralize ReAct conversation reset state to keep route reset and manual clear behavior consistent

Fixes #3075

Tests

• node integration check for ReAct sidebar reset/refresh wiring
• node behavior check for react-agent event helper subscribe/dispatch/unsubscribe
• npx eslint components/layout/side-bar.tsx pages/index.tsx utils/react-agent-events.ts
• npx prettier --check components/layout/side-bar.tsx pages/index.tsx utils/react-agent-events.ts
• npx tsc --pretty false --noEmit --target es6 --lib dom,esnext --strict --module esnext --moduleResolution node --skipLibCheck utils/react-agent-events.ts
• npm run build (compiled successfully, then failed on existing unrelated Prettier issue in components/chat/chat-content/ReferencesContent.tsx)

Notes

• Full npm ci is currently blocked because package-lock.json is out of sync with package.json; yarn --frozen-lockfile is also blocked by the current yarn.lock state. I installed local dependencies with package-lock writes disabled only for verification.

[wolfkill]

Additional compatibility hardening pushed in 11af787:

• New Task / Clear Chat now aborts any in-flight ReAct stream before resetting state.
• Stale SSE callbacks are ignored after a reset, so an old response cannot repopulate a fresh task.
• The summary typing interval also stops after reset.

Extra verification:

• compat event helper checks passed
• compat wiring checks passed
• npx eslint components/layout/side-bar.tsx pages/index.tsx utils/react-agent-events.ts
• npx prettier --check components/layout/side-bar.tsx pages/index.tsx utils/react-agent-events.ts
• npx tsc --pretty false --noEmit --target es6 --lib dom,esnext --strict --module esnext --moduleResolution node --skipLibCheck utils/react-agent-events.ts
• git diff --check
• npx next build --no-lint (compiled successfully and generated 52 static pages)