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/activity-rover-theremin.mdx

@@ -1,8 +1,10 @@
 ---
 title: "Activity: Rover Theremin"
 date: "2024-12-31"
-categories: ["Activities", "All", "Simulator Activities"]
+categories: ["Activities", "Simulator Activities"]
 tags: ["Colour Sensors", "Branching", "Iteration", "Algorithm Design", "Advanced"]
+codingSkills: ["Branching", "Iteration", "Algorithm Design"]
+roverConcepts: ["Colour"]
 excerpt: "Let’s convert the rover into one of the weirdest musical instruments of all time! In this activity, you will learn to code the rover to act like a Theremin and play it without touching a thing. Getting Into Music: As instruments go, the"
 featuredImage: "/images/resources/activity-rover-theremin.jpg"
 ---
@@ -43,8 +45,6 @@ As seen in the code, if the left IR distance sees less than 6cm, the code will c
 
 Now, all that is left is to add a large 'if/else if 'code to the inside of the volume control to control the pitch. As the code shows, the 'if' statement checks through each distance on the note scale to see which note is being triggered. The code plays the correct note depending on how far the hand is from the right IR.
 
-![](/images/content/d68678-code.png)
-
 #### Tips:
 
 Try to make your own note scale and change the code to suit it. Maybe you can add two octaves of notes on your page.