Abort (#1924)
* Add AbortRegistry for user-initiated generation cancellation
Introduces an AbortRegistry singleton to track and manage AbortControllers for active conversation generations, enabling user-initiated cancellation of ongoing requests. Updates text generation and endpoint logic to support abort signals, and ensures proper cleanup and partial response handling when a generation is aborted. The stop-generating endpoint now triggers aborts via the registry.
* Remove 'continue message' feature from chat flow
Eliminated the 'continue message' functionality and related code, including the ContinueBtn component, associated props, and logic in both frontend and backend. This simplifies the message handling flow and removes unused parameters and UI elements.
