Refine AI Essentials navigation: add Core Concepts dropdown and remove tool mini-badges

[JimWentworth]

Motivation

• Separate the interactive activities from the terminology reference by surfacing core concept activities in a dedicated menu next to the terms page.
• Prepare navigation to support multiple categories (e.g., Core Concepts now, Teaching/Learning later) while keeping the overview content for discovery.
• Simplify the activity headers by removing the small green tool identifier pills which are not needed.

Description

• Split navigation data in shared.js into pages and coreConcepts, and add a Core Concepts dropdown (.nav-group, .nav-dropdown, .nav-dropdown-toggle) that populates links and preserves completion indicators via data-activity-link.
• Add dropdown styling and related CSS tweaks in ai-essentials/styles.css, and remove the .tool-mini-badge styling.
• Remove the inline tool mini-badge elements from the three activity pages (activity-prompts-temperature.html, activity-hallucinations-rag.html, activity-tokens-context.html).
• Preserve existing completion tracking behavior based on localStorage (aiEssentialsProgress) and ensure the dropdown links reflect completion state.

Testing

• Launched a local server with python -m http.server 8000 and ran a Playwright script to load terminology.html and capture a screenshot (artifacts/ai-essentials-terminology-nav.png), which completed successfully.
• No unit or integration test suite was added for these static HTML/CSS/JS changes.

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Codex Task