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

phenomena.html

@@ -44,7 +44,7 @@
     <div class="content-section clearfix">
 
       <div class="figure figure-right">
-        <img src="https://images.unsplash.com/photo-1534274988757-a28bf1a94c3b?w=600&h=400&fit=crop" alt="Dramatic lightning storm over a dark landscape">
+        <img src="photos/photo-1429552077091-836152271555_600x400.jpg" alt="Dramatic lightning storm over a dark landscape">
         <p class="figure-caption">Lightning in its ordinary form is terrifying enough. Ball lightning is something else entirely. <small>Photo: Unsplash</small></p>
       </div>
 
@@ -72,7 +72,7 @@
     <div class="content-section clearfix">
 
       <div class="figure figure-left">
-        <img src="https://images.unsplash.com/photo-1576091160399-112ba8d25d1d?w=600&h=400&fit=crop" alt="Close-up of a medical pill in hand">
+        <img src="photos/1576091160399-112ba8d25d1d_600x400.jpg" alt="Close-up of a medical pill in hand">
         <p class="figure-caption">A sugar pill, prescribed with authority, can relieve real pain. The placebo effect is not imaginary — it is physiological. <small>Photo: Unsplash</small></p>
       </div>
 
@@ -144,7 +144,7 @@
     <div class="content-section clearfix">
 
       <div class="figure figure-right">
-        <img src="https://images.unsplash.com/photo-1484291150161-469ed51c68a2?w=600&h=400&fit=crop" alt="Ferrofluid forming dramatic spiky patterns on a glass surface under magnetic field">
+        <img src="photos/photo-1653163061406-730a0df077eb_600x400.jpg" alt="Ferrofluid forming dramatic spiky patterns on a glass surface under magnetic field">
         <p class="figure-caption">A ferrofluid responds to a magnet by forming spiky towers — each one a balance between magnetic force and surface tension. <small>Photo: Unsplash</small></p>
       </div>
 
@@ -206,7 +206,7 @@
     <div class="content-section clearfix">
 
       <div class="figure figure-left">
-        <img src="https://images.unsplash.com/photo-1464822759023-fed622ff2c3b?w=600&h=400&fit=crop" alt="Dramatic rocky landscape at night with mysterious atmospheric glow">
+        <img src="photos/1464822759023-fed622ff2c3b_600x400.jpg" alt="Dramatic rocky landscape at night with mysterious atmospheric glow">
         <p class="figure-caption">Earthquake lights have been reported for centuries — luminous phenomena that appear in the sky before or during seismic events. <small>Photo: Unsplash</small></p>
       </div>
 
@@ -239,7 +239,7 @@
       <p>The mystery was finally solved in 2014, when a team led by paleoclimatologist Richard Norris placed GPS-equipped stones on the playa and set up a time-lapse camera system. What they captured was extraordinary. On winter nights, a thin layer of ice forms on the shallow ephemeral pond that covers part of the playa. As the sun rises and the ice begins to break up, the wind pushes large sheets of ice — and the ice sheets push the rocks embedded within them, causing them to slide slowly across the muddy surface at speeds of roughly 3 to 5 meters per minute. The trails are carved by the rocks dragging through the soft sediment beneath the water. The solution was elegant, but the phenomenon itself remains no less wondrous: to see it, you need perfect conditions — water, ice, sun, and wind — all arriving in the right sequence. <span class="easter-egg">The playa knows the schedule, even if we do not.</span></p>
 
       <div class="fact-box">
-        <img src="img/ammonite-shell-fossil-image.png" alt="" style="float:right; margin: 0 0 8px 12px; max-width:60px;" class="clipart">
+        <img src="img/ammonite-shell-image.png" alt="" style="float:right; margin: 0 0 8px 12px; max-width:60px;" class="clipart">
         <h4>By the Numbers: Racetrack Playa</h4>
         <ul class="fact-list">
           <li><strong>Elevation:</strong> 1,132 m (3,714 ft) above sea level</li>