dashboard: persist sprint activity feed across server restarts

[trsdn]

Summary

The Sprint view activity feed becomes empty after the dashboard/server restarts, even when the sprint has already produced a long execution history.

Reproduction

1. Run a sprint until the Sprint view shows multiple activity entries.
2. Restart the dashboard process (web command / ws-server).
3. Re-open the same sprint in the dashboard.
4. Observe that the Sprint view activity feed is empty, while the file logs still contain the full history.

Expected

The Sprint view should rehydrate historical activity for the selected sprint after reconnect/restart, or the server should replay persisted sprint history so the activity feed remains useful for paused/failed sprint diagnosis.

Actual

Only live, in-memory buffered events are replayed. After a restart, the event buffer is empty, so the frontend shows no activity history for the sprint.

Root cause

• src/dashboard/ws-server.ts replays eventBuffer only from the current process lifetime.
• src/dashboard/frontend/src/store.ts clears activities on websocket close/reconnect.
• There is no persisted activity/history replay path for the Sprint view, even though sprint state and file logs survive restarts.

Impact

This makes paused/failed sprint forensics much harder. Example: Sprint 228 in QuantumEdgeTraderNG retained the disk logs and state, but the Sprint view activity pane was blank after the dashboard restart.