feat: add topic filter dropdown in course schedule and detail page

[ahmed-arb]

Add Topic Filter Dropdown to Course Schedule and Details Page

Summary

This PR adds a new "Topic" dropdown to the Course Schedule and Details page, allowing course authors to select a topic category for their course from a list of available options.

Changes

UI Components

• DetailsSection Component: Added topic dropdown alongside the existing language dropdown
  • New topic and topicOptions props
  • formattedTopic() helper function to display selected topic or placeholder
  • Dropdown renders all available topics from the backend
  • New SearchableCreatableDropdown component

Styling

• DetailsSection.scss: Added .dropdown-topic styles with scrollable menu (max height: 20rem) to match language dropdown behavior

Data Flow

• index.jsx:
  • Added topicOptions from settings API response
  • Added topic to course details state
  • Passed both props to DetailsSection component
  • Topic selection handled via existing handleValuesChange callback

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Related PRs:
edx-platform: wikimedia/edx-platform#581
openedx-wikilearn-features: wikimedia/openedx-wikilearn-features#17

Ticket: wikimedia/edx-platform#569