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

inventions.html

@@ -41,7 +41,7 @@
 
     <div class="content-section clearfix">
       <div class="figure-right">
-        <img src="https://images.unsplash.com/photo-1581093458791-9d42e3c7e117?w=600&h=400&fit=crop" alt="Gears and mechanisms of an antique machine">
+        <img src="photos/photo-1633451238042-85d93d267866_600x400.jpg" alt="Gears and mechanisms of an antique machine">
         <p class="figure-caption">A mechanism from the Industrial Revolution — when humanity learned to harness the forces of steam and iron.</p>
       </div>
       <p class="drop-cap">Every invention begins with a gap between what is and what could be. Someone, somewhere, looks at the world and thinks: <em>there must be a better way.</em> Sometimes that thought leads to decades of painstaking labour. Other times it arrives by accident — a spilled chemical, an overheated candy bar, a piece of adhesive that doesn't quite stick. This chapter is about both kinds: the inventions that were pursued with ferocious intent and the ones that stumbled into existence through sheer luck. Both sorts have shaped the world you live in, though the accidental ones tend to have the better stories.</p>
@@ -61,7 +61,7 @@
       <h3>Penicillin: The Mould That Saved Millions</h3>
       <div class="clearfix">
         <div class="figure-left">
-          <img src="https://images.unsplash.com/photo-1582719471384-894fbb16a07a?w=600&h=400&fit=crop" alt="Laboratory glassware and Petri dishes">
+          <img src="photos/photo-1617155092918-480ef0b17330_600x400.jpg" alt="Laboratory glassware and Petri dishes">
           <p class="figure-caption">Alexander Fleming's untidy laboratory — where a stray spore of Penicillium notatum changed medicine forever.</p>
         </div>
         <p>In September 1928, the Scottish bacteriologist Alexander Fleming returned from holiday to his cramped laboratory at St. Mary's Hospital in London. He had been studying staphylococci bacteria, but he had a habit of leaving his Petri dishes in a disorderly pile before going away. When he examined the dishes, he noticed something startling: a patch of mould had grown on one of them, and around that mould, the bacteria had been destroyed.</p>
@@ -89,10 +89,10 @@
     <div class="content-section">
       <h2>The Printing Press &amp; the Shape of the World</h2>
 
-      <img src="img/old-leather-book-illustration.png" alt="" class="clipart-left">
+      <img src="img/old-leather-book-search-icon.png" alt="" class="clipart-left">
 
       <div class="did-you-know">
-        <img src="img/quill-and-ink-jar-clip.png" alt="" style="float:right; margin: 0 0 8px 12px; max-width:60px;" class="clipart">
+        <img src="img/ornate-blue-egg-clip-art.png" alt="" style="float:right; margin: 0 0 8px 12px; max-width:60px;" class="clipart">
         <strong>Did You Know?</strong><br>
         Before the printing press, a single hand-copied Bible could take a scribe two to three years to complete and cost more than a house. Within fifty years of Gutenberg's invention, an estimated 20 million volumes had been printed in Europe. The price of a book fell by more than 80%. It remains the single most dramatic price drop in the history of information.
       </div>
@@ -137,7 +137,7 @@
       <img src="img/lit-gold-bulb-icon.png" alt="" class="clipart-left">
 
       <div class="figure-right">
-        <img src="https://images.unsplash.com/photo-1513506003901-1e6a229e2d15?w=600&h=400&fit=crop" alt="A glowing vintage lightbulb">
+        <img src="photos/1513506003901-1e6a229e2d15_600x400.jpg" alt="A glowing vintage lightbulb">
         <p class="figure-caption">The incandescent lamp — not the invention of one man, but the product of decades of collective problem-solving.</p>
       </div>
 
@@ -186,7 +186,7 @@
 
     <div class="content-section">
       <div class="figure">
-        <img src="https://images.unsplash.com/photo-1581092918056-0c4c3acd3789?w=900&h=350&fit=crop" alt="An intricate clockwork mechanism">
+        <img src="photos/1581092918056-0c4c3acd3789_900x350.jpg" alt="An intricate clockwork mechanism">
         <p class="figure-caption">The interior of a 19th-century clockwork mechanism — every gear a tiny argument against chaos.</p>
       </div>
     </div>
@@ -245,4 +245,4 @@
 
   <script src="js/main.js"></script>
 </body>
-</html>
+</html>