fix(markdown): formatting inside bold/italic/strike, monochrome inline code

Bold, italic and strikethrough render their inside again, so **`code`**
shows bold code instead of backticks; inline code uses the theme's code
background with the main text colour instead of the accent colour.

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

frontend/src/lib/__tests__/markdown.test.tsx

@@ -10,6 +10,13 @@ describe('renderInlineMarkdown', () => {
     expect(strong?.textContent).toBe('fettem');
   });
 
+  it('renders inline code inside bold text', () => {
+    const { container } = render(<div>{renderInlineMarkdown('Der Block **`services`** startet')}</div>);
+    const code = container.querySelector('strong code');
+    expect(code?.textContent).toBe('services');
+    expect(container.textContent).toBe('Der Block services startet');
+  });
+
   it('renders italic text with <em> tag', () => {
     const { container } = render(<div>{renderInlineMarkdown('Text mit *kursivem* Wort')}</div>);
     const em = container.querySelector('em');