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/ultrasonic-sensor.mdx

@@ -1,7 +1,7 @@
 ---
 title: "The Ultrasonic Sensor"
 date: "2021-07-21"
-categories: ["All", "Sensor Guides"]
+categories: ["Sensor Guides"]
 tags: []
 excerpt: "Learn all about the ultrasonic sensor!"
 featuredImage: "/images/resources/ultrasonic-sensor.png"
@@ -33,8 +33,6 @@ To detect objects, the ultrasonic sensor uses sound. Ultrasonic sound is defined
 
 **Based on how long it took the sound to bounce back, the sensor calculates how far away the object is using the speed of sound.**
 
-![](/images/content/65df02-ultrasonic-animation.gif)
-
 **The Parts That Make It Work** The sensor is made up of two different components,** the transmitter** and ** the receiver**. These are called transducers. The transmitter is making the ultrasonic sound and the receiver is listening for the reflected sound waves.  Some ultrasonic sensors use a single transducer as both the transmitter and receiver.
 
 ![](/images/content/aaa95f-sensor-detector.png)