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/lib/components/recipes/CardAdd.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
 
 import Cross from '$lib/assets/icons/Cross.svelte'
+import { toast } from '$lib/js/toast.svelte'
 import "$lib/css/shake.css"
 import "$lib/css/icon.css"
 import { onMount } from 'svelte'
@@ -32,14 +33,14 @@ function handleFileSelect(event: Event) {
 
 	// Validate MIME type
 	if (!ALLOWED_MIME_TYPES.includes(file.type)) {
-		alert('Invalid file type. Please upload a JPEG, PNG, or WebP image.');
+		toast.error('Invalid file type. Please upload a JPEG, PNG, or WebP image.');
 		input.value = '';
 		return;
 	}
 
 	// Validate file size
 	if (file.size > MAX_FILE_SIZE) {
-		alert(`File too large. Maximum size is 5MB. Your file is ${(file.size / 1024 / 1024).toFixed(2)}MB.`);
+		toast.error(`File too large. Maximum size is 5MB. Your file is ${(file.size / 1024 / 1024).toFixed(2)}MB.`);
 		input.value = '';
 		return;
 	}