feat(workspace): deleted folders go to the trash as a whole, after a confirmation

A deleted personal folder is kept in trashed_folders with its subfolders;
its notes are trashed and tied to it. The trash lists the folder as one
entry with its note count; restoring brings back the folder tree (empty
subfolders too) and all notes, deleting for good, emptying the trash and
the notes trash delay remove it with its notes. Folders in shared folders
keep the old way.

Deleting a folder now asks first, in a small popover at the click
(ConfirmPopover, documented in ui-patterns).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/__tests__/ConfirmPopover.test.tsx

@@ -0,0 +1,25 @@
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/react';
+import { ConfirmPopover } from '../ConfirmPopover';
+
+describe('ConfirmPopover', () => {
+  it('confirms with its button, which has the focus (Enter)', () => {
+    const onConfirm = vi.fn();
+    render(<ConfirmPopover x={100} y={100} message="Ordner löschen?" confirmLabel="In den Papierkorb" onConfirm={onConfirm} onCancel={vi.fn()} />);
+    const button = screen.getByRole('button', { name: 'In den Papierkorb' });
+    expect(document.activeElement).toBe(button);
+    fireEvent.click(button);
+    expect(onConfirm).toHaveBeenCalledTimes(1);
+  });
+
+  it('closes without confirming on Abbrechen, Escape or a click outside', () => {
+    const onConfirm = vi.fn();
+    const onCancel = vi.fn();
+    render(<ConfirmPopover x={100} y={100} message="Ordner löschen?" confirmLabel="Löschen" onConfirm={onConfirm} onCancel={onCancel} />);
+    fireEvent.click(screen.getByRole('button', { name: 'Abbrechen' }));
+    fireEvent.keyDown(document, { key: 'Escape' });
+    fireEvent.pointerDown(document.body);
+    expect(onCancel).toHaveBeenCalledTimes(3);
+    expect(onConfirm).not.toHaveBeenCalled();
+  });
+});