Show upcoming workflow state immediately on action click

[Neokil]

Problem

After clicking an action button (e.g. "Approve" on an Investigation step), the UI waits for the server to confirm the state transition before updating the timeline. This introduces a noticeable delay and gives no immediate feedback that the action was registered, making the interface feel unresponsive.

Solution

1. On action button click, optimistically append the next state to the timeline immediately.
2. Switch the active view to the new state and show a "running" indicator in the body.
3. When the SSE event for the transition arrives, replace the optimistic state with the confirmed server state.
4. If the action fails, roll back the optimistic update and surface an error.