Site audit: fix broken images, add activity metadata, clean up content

- Remove broken image references from 25 MDX files (deleted spacer/content images)
- Add codingSkills and roverConcepts frontmatter to 24 activity pages
- Fix "Activity Demonstration" headings in 15 activity files (plain text -> h3)
- Remove unused "All" category from 89 MDX files
- Decode HTML entities (> <  ) in 9 content files
- Escape bare < > characters in MDX to fix build errors
- Standardize contact email to contact@micromelon.com.au
- Add missing description to rover-repair-request metadata
- Include previous session work: about-us updates, difficulty filtering,
  LearningPathway component, expanded schools list, new product/resource images

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

content/resources/setting-up-a-school.mdx

@@ -1,7 +1,7 @@
 ---
 title: "How To Set Up A School"
 date: "2021-06-14"
-categories: ["All", "Getting Started"]
+categories: ["Getting Started"]
 tags: []
 excerpt: "Learn how to set up your school through the teacher dashboard in the Micromelon Code Editor."
 featuredImage: "/images/resources/setting-up-a-school.png"
@@ -193,8 +193,6 @@ For this method;
 
 - Your students have the Code Editor open and are ready to log into their Micromelon account or create a new account.
 
-![](/images/content/e1773a-quick-join-code.png)
-
 #### What is a Quick Join Code?
 
 Every class has a unique 9 digit Quick Join Code. You can share this with your students and they can use it to join your class & school without needing and invitation.