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

frontend/icons/icon-512x512.svg

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="512" height="512" viewBox="0 0 512 512" 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="512" height="512" rx="76.8" fill="url(#grad1)"/>
+  
+  <!-- Book icon -->
+  <g transform="translate(128.0, 102.4)">
+    <!-- Book cover -->
+    <rect x="0" y="0" width="256.0" height="307.2" rx="10.24" fill="white" opacity="0.95"/>
+    
+    <!-- Book spine -->
+    <rect x="25.6" y="0" width="10.24" height="307.2" fill="#1a1a1a" opacity="0.2"/>
+    
+    <!-- Book pages lines -->
+    <line x1="51.2" y1="76.8" x2="230.4" y2="76.8" stroke="#1a1a1a" stroke-width="5.12" opacity="0.3"/>
+    <line x1="51.2" y1="128.0" x2="230.4" y2="128.0" stroke="#1a1a1a" stroke-width="5.12" opacity="0.3"/>
+    <line x1="51.2" y1="179.2" x2="230.4" y2="179.2" stroke="#1a1a1a" stroke-width="5.12" opacity="0.3"/>
+    <line x1="51.2" y1="230.4" x2="179.2" y2="230.4" stroke="#1a1a1a" stroke-width="5.12" opacity="0.3"/>
+  </g>
+</svg>