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

frontend/style.css

@@ -4088,3 +4088,121 @@ body.popup-mode .content-area {
     padding-left: 6px;
   }
 }
+
+/* ===== PWA STYLES ===== */
+
+/* PWA Update Notification */
+.pwa-update-notification {
+  position: fixed;
+  bottom: 20px;
+  right: 20px;
+  z-index: 10000;
+  animation: slideInUp 0.3s ease-out;
+}
+
+.pwa-update-content {
+  background: var(--accent);
+  color: white;
+  padding: 16px 20px;
+  border-radius: var(--radius);
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.3);
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  max-width: 400px;
+}
+
+.pwa-update-content span {
+  flex: 1;
+  font-size: 0.9rem;
+  font-weight: 500;
+}
+
+.pwa-update-btn {
+  background: white;
+  color: var(--accent);
+  border: none;
+  padding: 8px 16px;
+  border-radius: var(--radius);
+  font-size: 0.85rem;
+  font-weight: 600;
+  cursor: pointer;
+  transition: all 0.2s ease;
+  white-space: nowrap;
+}
+
+.pwa-update-btn:hover {
+  transform: translateY(-1px);
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.2);
+}
+
+.pwa-update-dismiss {
+  background: transparent;
+  border: none;
+  color: white;
+  font-size: 1.5rem;
+  line-height: 1;
+  cursor: pointer;
+  padding: 0;
+  width: 24px;
+  height: 24px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  opacity: 0.8;
+  transition: opacity 0.2s ease;
+}
+
+.pwa-update-dismiss:hover {
+  opacity: 1;
+}
+
+/* PWA Install Button (optional - can be added to header) */
+#pwa-install-btn {
+  display: none;
+  background: var(--accent);
+  color: white;
+  border: none;
+  padding: 8px 16px;
+  border-radius: var(--radius);
+  font-size: 0.85rem;
+  font-weight: 600;
+  cursor: pointer;
+  transition: all 0.2s ease;
+}
+
+#pwa-install-btn:hover {
+  background: var(--accent-green);
+  transform: translateY(-1px);
+}
+
+/* Animations */
+@keyframes slideInUp {
+  from {
+    transform: translateY(100%);
+    opacity: 0;
+  }
+  to {
+    transform: translateY(0);
+    opacity: 1;
+  }
+}
+
+/* Mobile responsive */
+@media (max-width: 768px) {
+  .pwa-update-notification {
+    bottom: 10px;
+    right: 10px;
+    left: 10px;
+  }
+  
+  .pwa-update-content {
+    padding: 12px 16px;
+    font-size: 0.85rem;
+  }
+  
+  .pwa-update-btn {
+    padding: 6px 12px;
+    font-size: 0.8rem;
+  }
+}