feat: add toast notification system, replace all alert() calls

Create shared toast store and Toast component mounted in root layout.
Wire toast.error() into all fitness API calls that previously failed
silently, and replace all alert() calls across recipes and cospend.

src/routes/+layout.svelte

@@ -1,6 +1,7 @@
 <script>
 	import '../app.css';
 	import { onNavigate } from '$app/navigation';
+	import Toast from '$lib/components/Toast.svelte';
 	let { children } = $props();
 
 	onNavigate((navigation) => {
@@ -20,4 +21,5 @@
 	});
 </script>
 
-{@render children()}
+{@render children()}
+<Toast />