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

space.html

@@ -44,7 +44,7 @@
       <p class="drop-cap">If the Sun were shrunk to the size of a basketball, the Earth would be a tiny peppercorn sitting about 26 metres away. Jupiter, the largest planet, would be a golf ball roughly 130 metres down the road. And the nearest star, Proxima Centauri? It would be another basketball roughly 6,800 kilometres away — across an ocean, on another continent. Space is not mostly empty. Space is almost entirely nothing, and the distances between even the closest things are almost impossible to think about without getting dizzy.</p>
 
       <div class="figure">
-        <img src="https://images.unsplash.com/photo-1462331940025-496dfbfc7564?w=600&h=400&fit=crop" alt="A vast spiral galaxy against the blackness of deep space">
+        <img src="photos/1462331940025-496dfbfc7564_600x400.jpg" alt="A vast spiral galaxy against the blackness of deep space">
         <p class="figure-caption">The Andromeda Galaxy, our nearest large galactic neighbour, contains roughly one trillion stars. It is 2.5 million light-years away — meaning the light arriving tonight left before Homo sapiens had even evolved. <em>Photo: Unsplash.</em></p>
       </div>
 
@@ -79,7 +79,7 @@
       <img src="img/gemstone-mineral-collection.png" alt="" class="clipart-left">
 
       <div class="figure-right clearfix">
-        <img src="https://images.unsplash.com/photo-1534796636912-3b95b3ab5986?w=600&h=400&fit=crop" alt="A brilliant blue-white star remnant surrounded by an ethereal nebula shell">
+        <img src="photos/1534796636912-3b95b3ab5986_600x400.jpg" alt="A brilliant blue-white star remnant surrounded by an ethereal nebula shell">
         <p class="figure-caption">A neutron star is the compressed core of a massive star that has exploded. It spins, it beams, it distorts the very fabric of spacetime around it. <em>Photo: Unsplash.</em></p>
       </div>
 
@@ -129,7 +129,7 @@
       <img src="img/old-fashioned-brass-telescope-image.png" alt="" class="clipart-right">
 
       <div class="figure-left clearfix">
-        <img src="https://images.unsplash.com/photo-1614732414444-096e22c3795e?w=600&h=400&fit=crop" alt="A gold-anodized record mounted on a spacecraft, shining against deep space">
+        <img src="photos/photo-1473646590311-c48e1bc77b44_600x400.jpg" alt="A gold-anodized record mounted on a spacecraft, shining against deep space">
         <p class="figure-caption">The Golden Record, mounted on each Voyager spacecraft, is etched with instructions for playback in scientific notation. If found, it would be the first audio ever heard from another civilisation. <em>Photo: Unsplash.</em></p>
       </div>
 
@@ -168,7 +168,7 @@
 
     <hr class="dk-rule-thin">
 
-    <img src="img/globe-cartoon-illustration.png" alt="" class="clipart-wide">
+    <img src="img/measuring-tool-measuring-tape-scale.png" alt="" class="clipart-wide">
 
     <div class="content-section">
       <h2>The Great Attractor</h2>
@@ -232,4 +232,4 @@
 
   <script src="js/main.js"></script>
 </body>
-</html>
+</html>