fix(web): stabilize chat runtime and paged history

[alexk-dev]

Summary

• move chat runtime and websocket lifecycle out of the page so active responses survive navigation and reconnect cleanly
• constrain chat scrolling to an internal container, add paged history loading on scroll-up, and preserve attachment-only messages in paged results
• normalize model labels and add backend/frontend regression coverage for runtime, paging, bind, reasoning, and recovery edge cases

Testing

• cd dashboard && npm run lint
• cd dashboard && npm test
• cd dashboard && npm run build
• ./mvnw -Dtest=WebSocketChatHandlerTest,SessionsControllerTest,DefaultHistoryWriterTest test
• ./mvnw test

[sonarqubecloud]

Quality Gate failed

Failed conditions
78.7% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud