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/sumo.mdx

@@ -1,8 +1,10 @@
 ---
 title: "Activity: Sumo I"
 date: "2021-06-03"
-categories: ["Activities", "All", "Simulator Activities"]
+categories: ["Activities", "Simulator Activities"]
 tags: ["Colour Sensors", "Branching", "Iteration", "Algorithm Design", "Ultrasonic", "Motors", "Beginner"]
+codingSkills: ["Branching", "Iteration", "Algorithm Design"]
+roverConcepts: ["Colour", "Ultrasonic", "Motors"]
 excerpt: "Learn the basics of sumo by programming your rover to stay in the ring and push opponents out using the colour and ultrasonic sensors."
 featuredImage: "/images/resources/sumo.png"
 ---