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/how-to-maintain-a-3d-printer.mdx

@@ -17,8 +17,6 @@ Note that this guide is focused on Creality FDM printers, however, the concepts
 
 Belts are usually used on 3D printers to move two or more of the axes. Belts must always be kept well-tensioned, and neglecting to maintain belt tension may lead to several issues in print quality. These issues include deformed dimensions, print surface defects, and layer shifting.
 
-![](/images/content/557a5b-layer-shifting.png)
-
 *Credit: pattertj (Reddit)*
 
 ![](/images/content/a4a35e-surface-distortion.png)
@@ -43,8 +41,6 @@ The image above shows that an eccentric nut is just a hexagonal nut with an offs
 
 Many printers will, at some point, come across the issue of a print unsticking from the build plate. The frequency of this occurrence can be significantly reduced by correctly levelling the build plate. The idea is to get the nozzle to the perfect height from the build plate across the entire surface.
 
-![](/images/content/d68b19-levelling.png)
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 *Credit: Raise3D*
 
 First, home the 3D printer, then disable the steppers (or turn the printer off). A great way to start levelling is to use a business card between the nozzle and the bed and tighten the nuts under the bed until you feel slight resistance when the card is moved. This process should be repeated twice. Then, an excellent way to properly fine-tune the levelling is to run a calibration print (some examples below).
@@ -95,7 +91,6 @@ A simple and effective way to store filament is to keep the rolls in an airtight
 
 ![](/images/content/82a358-dry-box.png)
 
-![](/images/content/15d396-black-spacer.png)
 
 ![](/images/content/6b505b-2021-12-16_16-01-19_1639630906.jpg-2_9aae3875-8e0b-41ff-8d46-14a38ce51916.webp.jpeg)