feat(workspace): copy bullets as Markdown, Ctrl+A twice selects the whole note

Ctrl+C over a selection of several bullets now puts Markdown on the
clipboard (lib/outlineMarkdown.ts, the format of the Markdown export:
tab indent, - [ ] to-dos, headings, callouts, fenced code); pasting it
into a note gives the same bullets again. A second Ctrl+A in a bullet
selects the whole note. Copying goes through the browser's copy event,
so it works on plain HTTP too.

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

frontend/src/lib/__tests__/outlineMarkdown.test.ts

@@ -0,0 +1,44 @@
+import { describe, it, expect } from 'vitest';
+import { blocksToMarkdown } from '../outlineMarkdown';
+import { parsePastedLines } from '../pasteOutline';
+import type { DocumentBlock } from '../types';
+
+const b = (type: DocumentBlock['type'], content: string, level = 0, extra: Partial<DocumentBlock> = {}): DocumentBlock => ({
+  id: Math.random().toString(36).slice(2),
+  type,
+  content,
+  level,
+  ...extra,
+});
+
+describe('blocksToMarkdown', () => {
+  it('writes a nested list with to-dos, headings and code', () => {
+    const md = blocksToMarkdown([
+      b('h2', 'Umzug', 0),
+      b('bullet', 'Server', 1),
+      b('todo', 'Backup prüfen', 2, { checked: true }),
+      b('todo', 'DNS umstellen', 2),
+      b('numbered', 'Erstens', 1),
+      b('code', 'docker compose up -d', 1, { lang: 'bash' }),
+    ]);
+    expect(md).toBe(
+      ['## Umzug', '', '- Server', '\t- [x] Backup prüfen', '\t- [ ] DNS umstellen', '1. Erstens', '```bash', 'docker compose up -d', '```'].join('\n')
+    );
+  });
+
+  it('starts a copied branch at the left margin', () => {
+    expect(blocksToMarkdown([b('bullet', 'Ast', 3), b('bullet', 'Kind', 4)])).toBe('- Ast\n\t- Kind');
+  });
+
+  it('pasted back, it gives the same bullets', () => {
+    // 'text' is the plain bullet of a note
+    const blocks = [b('text', 'Eins', 0), b('text', 'Zwei', 1), b('todo', 'Drei', 2, { checked: true }), b('text', 'Vier', 0)];
+    const items = parsePastedLines(blocksToMarkdown(blocks).split('\n'));
+    expect(items.map((i) => [i.type, i.content, i.indent, Boolean(i.checked)])).toEqual([
+      ['text', 'Eins', 0, false],
+      ['text', 'Zwei', 1, false],
+      ['todo', 'Drei', 2, true],
+      ['text', 'Vier', 0, false],
+    ]);
+  });
+});