pixel: add ensure fbc fallback

src/lib/conversion/client.ts

@@ -1,3 +1,4 @@
+import { getFbpFbc } from '../metapixel/fbc.ts';
 import type { TrackingManager } from '$lib/tracking.svelte';
 import type {
 	ConversionErrorResponseBody,
@@ -42,13 +43,12 @@ export class ConversionClient {
 		}
 	): Promise<ConversionResponseBody | ConversionErrorResponseBody> {
 		// Extract user data
-		const fbp = await cookieStore.get('_fbp');
-		const fbc = await cookieStore.get('_fbc');
+		const { fbp, fbc } = getFbpFbc();
 
 		const user: ConversionUserData = {
 			...options.user,
-			fbp: fbp?.value,
-			fbc: fbc?.value
+			fbp,
+			fbc
 		};
 
 		// Get event source URL & extract UTM params if present