Add TheConversation tracking

[jmsevin]

This pull request adds tracking for "conversation" corpus excerpts in the CardComponent, enabling integration with The Conversation's content tracker. It introduces a new externalId prop, ensures this prop is passed from parent components, and includes tests to verify the tracking logic. The most important changes are:

Conversation excerpt tracking:

• Added a trackConversationExcerpt method to CardComponent.vue that loads The Conversation's content tracker script when a "conversation" excerpt is opened and an externalId is present. The script is removed after loading or error. (src/components/CardComponent.vue, src/components/CardComponent.vueR29-R54)
• Updated the excerpt <details> element to trigger the new tracking method on toggle events. (src/components/CardComponent.vue, src/components/CardComponent.vueL103-R127)

Prop and type updates:

• Added a new optional externalId prop to CardComponent.vue and updated the Document type to include document_external_id. (src/components/CardComponent.vue, [1]; src/types/index.ts, [2]

Prop wiring in parent components:

• Passed the new externalId prop from parent components (SourcesListComponent.vue, SecondStep.vue) to CardComponent. (src/components/SourcesListComponent.vue, [1] [2]; src/components/tutor/SecondStep.vue, [3]

Testing:

• Added unit tests for the new tracking logic in CardComponent.spec.ts, covering correct loading and non-loading scenarios. (src/components/__tests__/CardComponent.spec.ts, src/components/tests/CardComponent.spec.tsR1-R64)