Issue #853: Fix gtag dataLayer push to use arguments instead of array (#885)

##### Description of Change

The gtag.js library expects `Arguments` objects in the `dataLayer`, not
arrays. Our implementation used rest params (`...args`) which pushed
arrays — gtag loaded but never sent data to Google.

Fix: switch to the canonical `function() { dataLayer.push(arguments); }`
pattern that Google's own snippet uses.

**Before:** gtag.js loaded (200 OK), zero `/g/collect` requests sent
**After:** gtag.js loaded, collection requests sent to Google Analytics

##### Related Issues

Follow-up fix for the GA4 instrumentation in #875

##### Type of Change

- [x] Bug fix (non-breaking change which fixes an issue)

##### Project Area(s) Affected

- [x] frontends/

##### Testing

- [x] Manual testing performed (Playwright verified `/g/collect`
requests)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: bjagg

frontends/lif_advisor_app/src/utils/analytics.ts

@@ -16,9 +16,10 @@ function loadGtag(): void {
   document.head.appendChild(script);
 
   window.dataLayer = window.dataLayer || [];
-  window.gtag = function gtag(...args: unknown[]) {
-    // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    (window.dataLayer as any[]).push(args);
+  // gtag.js expects the `arguments` object to be pushed into dataLayer (not a rest-parameter array)
+  // eslint-disable-next-line prefer-rest-params
+  window.gtag = function () {
+    window.dataLayer.push(arguments);
   };
   window.gtag('js', new Date());
   window.gtag('config', MEASUREMENT_ID, { anonymize_ip: true });

frontends/mdr-frontend/src/utils/analytics.ts

@@ -16,10 +16,9 @@ function loadGtag(): void {
   document.head.appendChild(script);
 
   window.dataLayer = window.dataLayer || [];
-  window.gtag = function gtag(...args: unknown[]) {
-    // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    (window.dataLayer as any[]).push(args);
-  };
+  // Must use `arguments` (not rest params) — gtag.js checks for Arguments objects in dataLayer
+  // eslint-disable-next-line prefer-rest-params
+  window.gtag = function () { window.dataLayer.push(arguments); };
   window.gtag('js', new Date());
   window.gtag('config', MEASUREMENT_ID, { anonymize_ip: true });
 }