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-whiteboard-marker-holder.mdx

@@ -22,25 +22,21 @@ Click on a button below to collect the print files!
 [Makerworld](https://makerworld.com/en/models/147862#profileId-641400)
 
 
-![](/images/content/38abce-spacer.png)
 
 ## Step 1:
 
 ![](/images/content/5d4455-step-1.png)
 
-![](/images/content/4a7361-spacer.png)
 
 ## Step 2
 
 ![](/images/content/b58ef1-step-2.png)
 
-![](/images/content/3efdb3-spacer.png)
 
 ## Complete
 
 ![](/images/content/dadb0a-final.jpg)
 
-![](/images/content/314de3-black-spacer.png)
 
 ## Servo Calibration:
 
@@ -50,15 +46,12 @@ Before attaching a servo horn to a servo, we need to make sure that the servo mo
 
 #### Step 1:
 
-![](/images/content/0ccc3b-code.png)
-
 Once step 1 is completed, plug the servo into the right port and run the following code:
 
 ![](/images/content/84663a-screenshot-2024-09-18-124443.jpg)
 
 Upload the code and attach the servo horn to the servo so it’s in this position when the servo is at 0.
 
-![](/images/content/15d396-black-spacer.png)
 
 ## What’s Next?