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/build-guide-ping-pong-shooter.mdx

@@ -24,55 +24,46 @@ Click on the buttons below to collect the print files!
 [Makerworld](https://makerworld.com/en/models/462433)
 
 
-![](/images/content/81cd07-spacer.png)
 
 ## Step 1:
 
 ![](/images/content/b2c12e-step-1.jpg)
 
-![](/images/content/f973e5-spacer.png)
 
 ## Step 2:
 
 ![](/images/content/a25c4d-step-2.jpg)
 
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 ## Step 3:
 
 ![](/images/content/428a39-step-3.jpg)
 
-![](/images/content/b63d82-spacer.png)
 
 ## Step 4:
 
 ![](/images/content/c85def-step-4.jpg)
 
-![](/images/content/b5ea5a-spacer.png)
 
 ## Step 5:
 
 ![](/images/content/d1ac6f-step-5.jpg)
 
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 ## Step 6:
 
 ![](/images/content/1b0721-step-6.jpg)
 
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 ## Step 7:
 
 ![](/images/content/f982f2-step-7.jpg)
 
-![](/images/content/9746aa-spacer.png)
 
 ## Complete:
 
 ![](/images/content/83539c-complete.png)
 
-![](/images/content/15d396-black-spacer.png)
 
 ## What’s Next?