Add UI animations: dialogs, tabs, settings, browser stagger, banner pulse

- Dialog modals: scale+fade entrance/exit with animated backdrop
- Tab panels: fade-in with subtle slide on switch
- Settings sections: content slide-down on expand
- Browser grid/list items: staggered cascade entrance animation
- Connection banner: slide-in + attention pulse on disconnect
- Accessibility: prefers-reduced-motion disables all animations

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

media_server/static/js/core.js

@@ -331,6 +331,14 @@ function showToast(message, type = 'success') {
     }, TOAST_DURATION_MS);
 }
 
+function closeDialog(dialog) {
+    dialog.classList.add('dialog-closing');
+    dialog.addEventListener('animationend', () => {
+        dialog.classList.remove('dialog-closing');
+        dialog.close();
+    }, { once: true });
+}
+
 function showConfirm(message) {
     return new Promise((resolve) => {
         const dialog = document.getElementById('confirmDialog');
@@ -344,7 +352,7 @@ function showConfirm(message) {
             btnCancel.removeEventListener('click', onCancel);
             btnConfirm.removeEventListener('click', onConfirm);
             dialog.removeEventListener('close', onClose);
-            dialog.close();
+            closeDialog(dialog);
         }
 
         function onCancel() { cleanup(); resolve(false); }