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/guide-to-roborave-line-follow.mdx

@@ -49,7 +49,7 @@ For the line-follow aspect of the challenge, the in-built colour sensors on the
 
 Because of this, the recommended technique is to build a tower of sorts on top of the Rover with a container to store the balls. This container should also have a system (like a servo) to release the balls when it reaches the tower. Our 3D-printed RoboRAVE Line-follow attachment encompasses these considerations. For more details, click on button below to head to the build guide.[Build Guide: Roborave Line Follow](https://micromelon.com.au/resources/build-guide-roborave-line-follow)
 
-![](/images/content/d08a3c-img_1131.jpg)
+![](/images/content/5400a3-img_1131.jpg)
 
 **Coding:**