Fix all top-level dev tasks treating engine as enabled (#6705)

# Description of Changes
Currently, `task dev` explicitly calls the backend with
`AIENGINE_ENABLED=true` even though it isn't being spawned, so you just
get a dead FAB in the UI. This PR fixes it so that the engine will only
be enabled for tasks that will actually spawn the engine.

It also fixes a bug with the chat which makes it unusable locally. The
API path was not going through `apiClient` so for local dev you end up
with `//api/v1/...` which is not a valid path, so you get CORS errors
when trying to connect to the AI engine.

Taskfile.yml

@@ -87,6 +87,7 @@ tasks:
         vars:
           PORT: '{{.BACKEND_PORT}}'
           AIENGINE_URL: 'http://localhost:{{.ENGINE_PORT}}'
+          AIENGINE_ENABLED: "true"
       - task: 'frontend:dev:{{.FRONTEND}}'
         vars:
           PORT: '{{.FRONTEND_PORT}}'