test(frontend): set up Vitest with jsdom and run tests in the image build

- Add vitest, jsdom and Testing Library as dev dependencies,
  replace the placeholder test:run script with vitest run.
- Add outliner tests for Backspace merging and caret placement on click.
- Run the frontend tests in the Dockerfile before the Vite build.
- Update docs (README, architecture, FLAW-19).

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

frontend/src/features/workspace/__tests__/OutlinerBlock.caret.test.tsx

@@ -0,0 +1,64 @@
+import React from 'react';
+import { describe, it, expect, afterEach } from 'vitest';
+import { render, fireEvent } from '@testing-library/react';
+import { OutlinerBlock, mapRenderedOffsetToRaw } from '../OutlinerBlock';
+import { DocumentBlock } from '../../../lib/types';
+
+describe('mapRenderedOffsetToRaw', () => {
+  it('is the identity for plain text', () => {
+    expect(mapRenderedOffsetToRaw('Hallo Welt', 'Hallo Welt', 4)).toBe(4);
+  });
+  it('skips bold markers', () => {
+    expect(mapRenderedOffsetToRaw('Hallo **Welt** du', 'Hallo Welt du', 8)).toBe(10);
+  });
+  it('maps behind closing markers', () => {
+    expect(mapRenderedOffsetToRaw('a **b** c', 'a b c', 4)).toBe(8);
+  });
+  it('maps into a link label', () => {
+    expect(mapRenderedOffsetToRaw('x [Seite](https://a.de) y', 'x Seite y', 4)).toBe(5);
+  });
+  it('handles the empty placeholder', () => {
+    expect(mapRenderedOffsetToRaw('', '​', 1)).toBe(0);
+  });
+});
+
+describe('clicking into an unfocused bullet', () => {
+  afterEach(() => {
+    delete (document as any).caretRangeFromPoint;
+  });
+
+  it('places the caret at the click position instead of the end', () => {
+    const block = { id: 'b1', type: 'bullet', content: 'Hallo **Welt** du', level: 0, position: 1 } as DocumentBlock;
+    const { container } = render(
+      <OutlinerBlock
+        docId="d"
+        block={block}
+        index={0}
+        isLast
+        maxAllowedLevel={1}
+        hasChildren={false}
+        isFocused={false}
+        canEdit
+        onUpdateBlock={() => {}}
+        onCreateBlockBelow={() => {}}
+        onDeleteBlock={() => {}}
+        onIndentBlock={() => {}}
+        onNavigateFocus={() => {}}
+        registerTextareaRef={() => {}}
+      />
+    );
+    const ta = container.querySelector('textarea') as HTMLTextAreaElement;
+    const strong = container.querySelector('strong') as HTMLElement; // rendered "Welt"
+    // jsdom has no layout: pretend the click hit between "We" and "lt"
+    (document as any).caretRangeFromPoint = () => {
+      const r = document.createRange();
+      r.setStart(strong.firstChild!, 2);
+      return r;
+    };
+
+    fireEvent.click(strong);
+
+    expect(document.activeElement).toBe(ta);
+    expect(ta.selectionStart).toBe(10); // "Hallo **We|lt** du"
+  });
+});

frontend/src/features/workspace/__tests__/OutlinerTree.merge.test.tsx

@@ -0,0 +1,79 @@
+import React, { useState } from 'react';
+import { describe, it, expect } from 'vitest';
+import { render, fireEvent, screen, act } from '@testing-library/react';
+import { OutlinerTree } from '../OutlinerTree';
+import { DocumentBlock } from '../../../lib/types';
+
+// Holds the latest block state rendered by the harness
+let live: DocumentBlock[] = [];
+
+const Harness: React.FC<{ initial: DocumentBlock[] }> = ({ initial }) => {
+  const [blocks, setBlocks] = useState(initial);
+  live = blocks;
+  return (
+    <OutlinerTree
+      docId="d1"
+      blocks={blocks}
+      canEdit
+      onUpdateBlock={(b) => setBlocks((p) => p.map((x) => (x.id === b.id ? b : x)))}
+      onUpdateBlocks={(u) => setBlocks((p) => p.map((x) => u.find((y) => y.id === x.id) || x))}
+      onCreateBlock={() => {}}
+      onDeleteBlock={(id) => setBlocks((p) => p.filter((x) => x.id !== id))}
+      onMoveBlock={() => {}}
+    />
+  );
+};
+
+const mk = (id: string, content: string, level = 0, type: any = 'bullet'): DocumentBlock =>
+  ({ id, type, content, level, position: 1 }) as DocumentBlock;
+
+const area = (text: string) => screen.getAllByDisplayValue(text)[0] as HTMLTextAreaElement;
+
+const backspaceAt = async (ta: HTMLTextAreaElement, pos = 0) => {
+  ta.focus();
+  ta.setSelectionRange(pos, pos);
+  await act(async () => {
+    fireEvent.keyDown(ta, { key: 'Backspace' });
+  });
+};
+
+describe('Backspace at the start of a bullet', () => {
+  it('appends its text to the previous bullet', async () => {
+    render(<Harness initial={[mk('a', 'Hallo '), mk('b', 'Welt')]} />);
+    await backspaceAt(area('Welt'));
+    expect(live.map((b) => b.content)).toEqual(['Hallo Welt']);
+  });
+
+  it('works for legacy blocks without a type', async () => {
+    render(<Harness initial={[mk('a', 'A', 0, undefined), mk('b', 'B', 0, '')]} />);
+    await backspaceAt(area('B'));
+    expect(live.map((b) => b.content)).toEqual(['AB']);
+  });
+
+  it('moves children up under the previous bullet', async () => {
+    render(<Harness initial={[mk('a', 'A'), mk('b', 'B'), mk('c', 'C', 1)]} />);
+    await backspaceAt(area('B'));
+    expect(live.map((b) => [b.content, b.level])).toEqual([
+      ['AB', 0],
+      ['C', 1],
+    ]);
+  });
+
+  it('does nothing when the caret is not at the start', async () => {
+    render(<Harness initial={[mk('a', 'A'), mk('b', 'BB')]} />);
+    await backspaceAt(area('BB'), 1);
+    expect(live).toHaveLength(2);
+  });
+
+  it('does not merge the first bullet', async () => {
+    render(<Harness initial={[mk('a', 'A'), mk('b', 'B')]} />);
+    await backspaceAt(area('A'));
+    expect(live).toHaveLength(2);
+  });
+
+  it('does not merge into a code block', async () => {
+    render(<Harness initial={[mk('a', 'x = 1', 0, 'code'), mk('b', 'B')]} />);
+    await backspaceAt(area('B'));
+    expect(live).toHaveLength(2);
+  });
+});