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/fitness/[stats=fitnessStats]/+page.svelte

@@ -5,6 +5,7 @@
 	import FitnessStreakAura from '$lib/components/fitness/FitnessStreakAura.svelte';
 	import { onMount } from 'svelte';
 	import { detectFitnessLang, fitnessSlugs, t } from '$lib/js/fitnessI18n';
+	import { toast } from '$lib/js/toast.svelte';
 
 	const lang = $derived(detectFitnessLang($page.url.pathname));
 
@@ -59,8 +60,11 @@
 				goalWeekly = d.weeklyWorkouts;
 				goalStreak = d.streak;
 				goalEditing = false;
+			} else {
+				const err = await res.json().catch(() => null);
+				toast.error(err?.error ?? 'Failed to save goal');
 			}
-		} finally {
+		} catch { toast.error('Failed to save goal'); } finally {
 			goalSaving = false;
 		}
 	}