Remove all hotlinked Unsplash images — download locally, replace 14 404s

- Downloaded all 31 Unsplash images to photos/ directory
- Found 14 URLs returning 404; replaced with working equivalents:
  about: book-room, hard-drives, forest-child
  human-body: digestive-system, human-eye
  inventions: gears (x2), lab-glassware
  maps/phenomena: old-map (x2), ornate-globe, earth-orbit
  natural-world: migrating-birds
  phenomena: lightning-storm, ferrofluid
  space: voyager-record
- Updated all 8 HTML files to reference local photos/ instead of Unsplash CDN
- Fixed broken ammonite clipart reference (fossil -> existing image)
- Updated CSS clipart hover effects and responsive rules

human-body.html

@@ -49,7 +49,7 @@
 
     <div class="content-section clearfix">
       <div class="figure figure-right">
-        <img src="https://images.unsplash.com/photo-1559757175-5700dde675bc?w=600&h=400&fit=crop" alt="The human brain, exterior view" width="600" height="400">
+        <img src="photos/1559757175-5700dde675bc_600x400.jpg" alt="The human brain, exterior view" width="600" height="400">
         <p class="figure-caption">The cerebral cortex alone contains an estimated 16 billion neurons, each connected to thousands of its neighbours by threadlike axons.</p>
       </div>
 
@@ -83,7 +83,7 @@
 
     <div class="content-section clearfix">
       <div class="figure figure-left">
-        <img src="https://images.unsplash.com/photo-1576086213369-97ed7c8431e2?w=600&h=400&fit=crop" alt="Human digestive system illustration" width="600" height="400">
+        <img src="photos/photo-1530213786676-41ad9f7736f6_600x400.jpg" alt="Human digestive system illustration" width="600" height="400">
         <p class="figure-caption">The enteric nervous system lines the full length of the gastrointestinal tract, from oesophagus to rectum.</p>
       </div>
 
@@ -137,7 +137,7 @@
 
     <div class="content-section clearfix">
       <div class="figure figure-right">
-        <img src="https://images.unsplash.com/photo-1584036561566-b93a901663f7?w=600&h=400&fit=crop" alt="Close-up of a human eye" width="600" height="400">
+        <img src="photos/photo-1483519173755-be893fab1f46_600x400.jpg" alt="Close-up of a human eye" width="600" height="400">
         <p class="figure-caption">The retina contains roughly 120 million rod cells and 6 million cone cells, each one a tiny photoreceiver tuned to particles of light.</p>
       </div>