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/activity-seed-planter.mdx

@@ -1,8 +1,10 @@
 ---
 title: "Activity: Seed Planter"
 date: "2023-10-31"
-categories: ["Activities", "All", "Simulator Activities"]
+categories: ["Activities", "Simulator Activities"]
 tags: ["Colour Sensors", "Branching", "Iteration", "Algorithm Design", "Advanced"]
+codingSkills: ["Branching", "Iteration", "Algorithm Design"]
+roverConcepts: ["Colour"]
 excerpt: "Use the seed planter attachment to start your own little garden. Organise your garden layout and have the rover precisely deposit seeds in soft soil. Cover it up as you go, and finish with some water! SetupFor this task, you must purcha"
 featuredImage: "/images/resources/activity-seed-planter.jpg"
 ---