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/accelerometer.mdx

@@ -1,7 +1,7 @@
 ---
 title: "The Accelerometer"
 date: "2021-06-22"
-categories: ["All", "Sensor Guides"]
+categories: ["Sensor Guides"]
 tags: []
 excerpt: "Learn what the accelerometer is and how to code it."
 featuredImage: "/images/resources/accelerometer.gif"
@@ -141,7 +141,7 @@ Let’s write a simple program to change our Rover’s LEDs depending on the val
 
 - Change the **X Axis** dropdown to ** Z Axis**.
 
-The python command for reading the Z axis is **IMU.readAccel(2)**- Change the second value on the **< condition** block from ** 10 to 0**. This will check if the value is either positive or negative.
+The python command for reading the Z axis is **IMU.readAccel(2)**- Change the second value on the **`<` condition** block from ** 10 to 0**. This will check if the value is either positive or negative.
 
 - Inside the IF/ELSE block true case, place an **Set All LEDs** block. You can find this in the ** Lights/Sound** category.