feat: add Progressive Web App (PWA) support with service worker registration, manifest, update notifications, and install prompts

frontend/icons/icon-72x72.svg

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="72" height="72" viewBox="0 0 72 72" xmlns="http://www.w3.org/2000/svg">
+  <defs>
+    <linearGradient id="grad1" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#2563eb;stop-opacity:1" />
+      <stop offset="100%" style="stop-color:#1d4ed8;stop-opacity:1" />
+    </linearGradient>
+  </defs>
+  
+  <!-- Background -->
+  <rect width="72" height="72" rx="10.799999999999999" fill="url(#grad1)"/>
+  
+  <!-- Book icon -->
+  <g transform="translate(18.0, 14.4)">
+    <!-- Book cover -->
+    <rect x="0" y="0" width="36.0" height="43.199999999999996" rx="1.44" fill="white" opacity="0.95"/>
+    
+    <!-- Book spine -->
+    <rect x="3.6" y="0" width="1.44" height="43.199999999999996" fill="#1a1a1a" opacity="0.2"/>
+    
+    <!-- Book pages lines -->
+    <line x1="7.2" y1="10.799999999999999" x2="32.4" y2="10.799999999999999" stroke="#1a1a1a" stroke-width="0.72" opacity="0.3"/>
+    <line x1="7.2" y1="18.0" x2="32.4" y2="18.0" stroke="#1a1a1a" stroke-width="0.72" opacity="0.3"/>
+    <line x1="7.2" y1="25.2" x2="32.4" y2="25.2" stroke="#1a1a1a" stroke-width="0.72" opacity="0.3"/>
+    <line x1="7.2" y1="32.4" x2="25.2" y2="32.4" stroke="#1a1a1a" stroke-width="0.72" opacity="0.3"/>
+  </g>
+</svg>