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/proprietary/hooks/useRequestHeaders.ts

@@ -0,0 +1,4 @@
+export function useRequestHeaders(): HeadersInit {
+  const token = localStorage.getItem('stirling_jwt');
+  return token ? { 'Authorization': `Bearer ${token}` } : {};
+}

frontend/src/proprietary/hooks/useShouldShowWelcomeModal.ts

@@ -0,0 +1,17 @@
+import { useMediaQuery } from '@mantine/hooks';
+import { usePreferences } from '@app/contexts/PreferencesContext';
+import { useAuth } from '@app/auth/UseSession';
+
+export function useShouldShowWelcomeModal(): boolean {
+  const { preferences } = usePreferences();
+  const { session, loading } = useAuth();
+  const isMobile = useMediaQuery("(max-width: 1024px)");
+
+  // Only show welcome modal if user is authenticated (session exists)
+  // This prevents the modal from showing on login screens when security is enabled
+  return !loading
+    && !preferences.hasCompletedOnboarding
+    && preferences.toolPanelModePromptSeen
+    && !isMobile
+    && !!session;
+}