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/spike-drop.mdx

@@ -1,17 +1,19 @@
 ---
 title: "Activity: Spike Drop"
 date: "2022-02-23"
-categories: ["All", "Simulator Activities"]
+categories: ["Simulator Activities"]
 tags: ["Colour Sensors", "Branching", "Iteration", "Ultrasonic", "Robot Simulator", "Beginner"]
+codingSkills: ["Branching", "Iteration"]
+roverConcepts: ["Colour", "Ultrasonic"]
 excerpt: "Simulator activity for learning ultrasonic, colour sensor and iteration."
-featuredImage: "/images/resources/spike-drop.png"
+featuredImage: "/images/resources/spike-drop.jpg"
 ---
 
 Find a way to drive the simulator rover into the green parking lot. Use your colour or ultrasonic sensor to help you find the safe zone. Watch out though, go to far and you’ll fall and get spiked!
 
 ![](/images/content/a252ca-spike-drop-demo.gif)
 
-Activity Demonstration
+### Activity Demonstration
 
 ### Setup