chore: replace SVG logo with transparent-background PNG for dark theme support

- Add new logo.png (2090x512) with transparent background
- Update AppLayout and LoginPage imports from .svg to .png
- Old SVG had opaque background that clashed with dark theme

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/AppLayout.tsx

@@ -18,7 +18,7 @@ import { usePreferencesStore } from '../../stores/preferencesStore';
 import { Sidebar } from './Sidebar';
 import { AppTour } from '../onboarding/AppTour';
 import { OnboardingWizard } from '../onboarding/OnboardingWizard';
-import logoSrc from '../../assets/logo.svg';
+import logoSrc from '../../assets/logo.png';
 
 export function AppLayout() {
   const [opened, { toggle, close }] = useDisclosure();

frontend/src/pages/auth/LoginPage.tsx

@@ -16,7 +16,7 @@ import { IconAlertCircle } from '@tabler/icons-react';
 import { useNavigate, Link } from 'react-router-dom';
 import api from '../../services/api';
 import { useAuthStore } from '../../stores/authStore';
-import logoSrc from '../../assets/logo.svg';
+import logoSrc from '../../assets/logo.png';
 
 export function LoginPage() {
   const [loading, setLoading] = useState(false);