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/showcase-rover-and-arduino-line-follow.mdx

@@ -11,7 +11,7 @@ Using I2C and an Arduino Nano, a QTR-8RC line sensor array can be attached to th
 
 An overview of the I2C protocol and how to use it with the Rover can be found here.
 
-![](/images/content/e33faa-img_1366.jpg)
+![](/images/content/19bff0-img_1366.jpg)
 
 ## The Sensor (QTR-8RC)
 
@@ -71,7 +71,7 @@ void readSensor(){
 
 ![](/images/content/5745b8-circuit.png)
 
-![](/images/content/66a722-rover-expansion-header.png)
+![](/images/content/0c946e-rover-pinout.png)
 
 ## The Rover