fix: delete confirm dialog never visible + race condition

- Add missing .confirm-dialog.active { display: flex; opacity: 1 } CSS rule.
  The confirm dialog was created with display:none and the .active class
  was added, but no CSS rule changed it to visible — so the user never
  saw the confirmation prompt and delete appeared to do nothing.
- Capture file/folder target before closeContextMenu in delete handlers
  to prevent null reference race condition (same as rename/share fix).

static/css/style.css

@@ -2489,6 +2489,11 @@ select:focus {
     animation: modalFadeIn 0.2s ease;
 }
 
+.confirm-dialog.active {
+    display: flex;
+    opacity: 1;
+}
+
 .confirm-dialog-content {
     background: white;
     border-radius: 16px;

static/js/contextMenus.js

@@ -70,13 +70,11 @@ const contextMenus = {
         });
 
         document.getElementById('delete-folder-option').addEventListener('click', async () => {
-            if (window.app.contextMenuTargetFolder) {
-                await window.fileOps.deleteFolder(
-                    window.app.contextMenuTargetFolder.id, 
-                    window.app.contextMenuTargetFolder.name
-                );
-            }
+            const folder = window.app.contextMenuTargetFolder;
             window.ui.closeContextMenu();
+            if (folder) {
+                await window.fileOps.deleteFolder(folder.id, folder.name);
+            }
         });
 
         // File context menu options
@@ -174,13 +172,11 @@ const contextMenus = {
         });
 
         document.getElementById('delete-file-option').addEventListener('click', async () => {
-            if (window.app.contextMenuTargetFile) {
-                await window.fileOps.deleteFile(
-                    window.app.contextMenuTargetFile.id,
-                    window.app.contextMenuTargetFile.name
-                );
-            }
+            const file = window.app.contextMenuTargetFile;
             window.ui.closeFileContextMenu();
+            if (file) {
+                await window.fileOps.deleteFile(file.id, file.name);
+            }
         });
 
         // Rename dialog events