feat(workspace): borderless code blocks, round code/audio/dictation blocks

Code blocks lose border and header bar: language at the top left, a round
copy button at the bottom right on hover; converting back to text moves to
the bullet's action bubble (also for numbered items). Code, audio and
dictation blocks use fixed round corners whatever radius the theme sets.

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

frontend/src/features/workspace/__tests__/CodeBlockEditor.test.tsx

@@ -77,19 +77,14 @@ describe('CodeBlockEditor', () => {
     expect(updates[0].lang).toBe('yaml');
     expect(select.value).toBe('yaml');
 
-    // "Als Text" converts the code block back to a bullet
-    fireEvent.click(screen.getByRole('button', { name: 'Als Text' }));
-    expect(updates[updates.length - 1]?.type).toBe('bullet');
-
     unmount();
 
-    // Reader view (!canEdit): shows plain text instead of <select> and hides "Als Text"
+    // Reader view (!canEdit): shows plain text instead of <select> 
     const readerView = render(
       <CodeBlockEditor block={makeCodeBlock({ lang: 'yaml' })} canEdit={false} />
     );
     expect(readerView.container.querySelector('select')).toBeNull();
     expect(readerView.getByText('yaml')).toBeTruthy();
-    expect(readerView.queryByText('Als Text')).toBeNull();
   });
 
   it('copies the code content to the clipboard and shows "Kopiert!" for 1.5s', () => {
@@ -108,14 +103,14 @@ describe('CodeBlockEditor', () => {
     fireEvent.click(copyBtn);
 
     expect(writeTextMock).toHaveBeenCalledWith('echo "hello world"');
-    expect(screen.getByText('Kopiert!')).toBeTruthy();
+    expect(screen.getByRole('button', { name: 'Kopiert!' })).toBeTruthy();
 
     act(() => {
       vi.advanceTimersByTime(1500);
     });
 
-    expect(screen.queryByText('Kopiert!')).toBeNull();
-    expect(screen.getByText('Kopieren')).toBeTruthy();
+    expect(screen.queryByRole('button', { name: 'Kopiert!' })).toBeNull();
+    expect(screen.getByRole('button', { name: 'Kopieren' })).toBeTruthy();
   });
 
   it('indents by 2 spaces on Tab and outdents on Shift+Tab without indenting the bullet', () => {