fix: track SPA navigations in Google Analytics

GA only fires on initial page load by default. Added $effect in layout
to send page_view events on client-side route changes (/ ↔ /play).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: andrewboldi

src/app.d.ts

@@ -0,0 +1,16 @@
+// See https://svelte.dev/docs/kit/types#app.d.ts
+// for information about these interfaces
+declare global {
+	namespace App {
+		// interface Error {}
+		// interface Locals {}
+		// interface PageData {}
+		// interface PageState {}
+		// interface Platform {}
+	}
+
+	// Google Analytics gtag
+	function gtag(...args: unknown[]): void;
+}
+
+export {};

src/routes/+layout.svelte

@@ -4,9 +4,18 @@
 	import 'katex/dist/katex.min.css';
 	import '../app.css';
 	import { settings } from '$lib/stores/settings';
+	import { page } from '$app/state';
 
 	let { children } = $props();
 
+	// Track SPA navigations in Google Analytics
+	$effect(() => {
+		const path = page.url.pathname;
+		if (typeof gtag === 'function') {
+			gtag('config', 'G-P98FJ92R7Z', { page_path: path });
+		}
+	});
+
 	function hexToRgb(hex: string): string {
 		const r = parseInt(hex.slice(1, 3), 16);
 		const g = parseInt(hex.slice(3, 5), 16);