Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/hooks/useShouldShowWelcomeModal.ts

@@ -0,0 +1,11 @@
+import { useMediaQuery } from '@mantine/hooks';
+import { usePreferences } from '@app/contexts/PreferencesContext';
+
+export function useShouldShowWelcomeModal(): boolean {
+  const { preferences } = usePreferences();
+  const isMobile = useMediaQuery("(max-width: 1024px)");
+
+  return !preferences.hasCompletedOnboarding
+    && preferences.toolPanelModePromptSeen
+    && !isMobile;
+}