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/utils/scarfTracking.ts

@@ -0,0 +1,95 @@
+/**
+ * Scarf analytics pixel tracking utility
+ *
+ * This module provides a firePixel function that can be called from anywhere,
+ * including non-React utility functions. Configuration and consent state are
+ * injected via setScarfConfig() which should be called from a React hook
+ * during app initialization.
+ *
+ * IMPORTANT: setScarfConfig() must be called before firePixel() will work.
+ * The initialization hook (useScarfTracking) is mounted in App.tsx.
+ *
+ * For testing: Use resetScarfConfig() to clear module state between tests.
+ */
+
+// Module-level state
+let configured: boolean = false;
+let enableScarf: boolean | null = null;
+let isServiceAccepted: ((service: string, category: string) => boolean) | null = null;
+let lastFiredPathname: string | null = null;
+let lastFiredTime = 0;
+
+/**
+ * Configure scarf tracking with app config and consent checker
+ * Should be called from a React hook during app initialization (see useScarfTracking)
+ *
+ * @param scarfEnabled - Whether scarf tracking is enabled globally
+ * @param consentChecker - Function to check if user has accepted scarf service
+ */
+export function setScarfConfig(
+  scarfEnabled: boolean | null,
+  consentChecker: (service: string, category: string) => boolean
+): void {
+  configured = true;
+  enableScarf = scarfEnabled;
+  isServiceAccepted = consentChecker;
+}
+
+/**
+ * Fire scarf pixel for analytics tracking
+ * Only fires if:
+ * - Scarf tracking has been initialized via setScarfConfig()
+ * - Scarf is globally enabled in config
+ * - User has accepted scarf service via cookie consent
+ * - Pathname has changed or enough time has passed since last fire
+ *
+ * @param pathname - The pathname to track (usually window.location.pathname)
+ */
+export function firePixel(pathname: string): void {
+  // Dev-mode warning if called before initialization
+  if (!configured) {
+    console.warn(
+      '[scarfTracking] firePixel() called before setScarfConfig(). ' +
+      'Ensure useScarfTracking() hook is mounted in App.tsx.'
+    );
+    return;
+  }
+
+  // Check if Scarf is globally disabled
+  if (enableScarf === false) {
+    return;
+  }
+
+  // Check if consent checker is available and scarf service is accepted
+  if (!isServiceAccepted || !isServiceAccepted('scarf', 'analytics')) {
+    return;
+  }
+
+  const now = Date.now();
+
+  // Only fire if pathname changed or it's been at least 250ms since last fire
+  if (pathname === lastFiredPathname && now - lastFiredTime < 250) {
+    return;
+  }
+
+  lastFiredPathname = pathname;
+  lastFiredTime = now;
+
+  const url = 'https://static.scarf.sh/a.png?x-pxid=3c1d68de-8945-4e9f-873f-65320b6fabf7'
+             + '&path=' + encodeURIComponent(pathname);
+
+  const img = new Image();
+  img.referrerPolicy = "no-referrer-when-downgrade";
+  img.src = url;
+}
+
+/**
+ * Reset scarf tracking configuration and state
+ * Useful for testing to ensure clean state between test runs
+ */
+export function resetScarfConfig(): void {
+  enableScarf = null;
+  isServiceAccepted = null;
+  lastFiredPathname = null;
+  lastFiredTime = 0;
+}