fix(polish): align Compose button label with sidebar nav

- HomeScreen header button: 'Compose note' → 'Compose' (matches sidebar)
- Updated tests to use exact name match and scoped queries to avoid
  matching the sidebar nav button

frontend/src/screens/HomeScreen.tsx

@@ -72,7 +72,7 @@ export function HomeScreen({ onNavigate, onCreateProfile }: HomeScreenProps) {
           </div>
           <Button variant="primary" onClick={() => onNavigate('compose')}>
             <Icon name="edit" size={18} />
-            Compose note
+            Compose
           </Button>
         </header>
 

frontend/src/test/HomeScreen.test.tsx

@@ -30,7 +30,7 @@ describe('HomeScreen', () => {
     expect(within(identityCard).getByText('Alice')).toBeInTheDocument();
     expect(within(identityCard).getByText(/npub1alice\.\.\./)).toBeInTheDocument();
 
-    const compose = screen.getByRole('button', { name: /Compose note/i });
+    const compose = screen.getByRole('button', { name: 'Compose' });
     await userEvent.setup().click(compose);
     expect(onNavigate).toHaveBeenCalledWith('compose');
   });

frontend/src/test/publishFlow.test.tsx

@@ -1,4 +1,4 @@
-import { render, screen } from '@testing-library/react';
+import { render, screen, within } from '@testing-library/react';
 import userEvent from '@testing-library/user-event';
 import App from '../App';
 import { createFakeBackend, installFakeBackend } from './fakeBackend';
@@ -13,7 +13,8 @@ describe('publication flow across screens', () => {
 
     await screen.findByRole('heading', { name: 'Home' });
 
-    await user.click(screen.getByRole('button', { name: /Compose note/i }));
+    const main = screen.getByRole('main');
+    await user.click(within(main).getByRole('button', { name: 'Compose' }));
     await screen.findByRole('heading', { name: 'Compose' });
 
     await user.type(screen.getByLabelText('Note content'), 'Hello from the flow test');