feat: auto-sync recipes and show sync button only in PWA mode

- Auto-sync recipes every 30 minutes when online in PWA mode
- Only show offline sync button when running as installed PWA
- Detect standalone mode via display-mode media query and iOS check
- Trigger initial sync on PWA install (appinstalled event)
- Listen for online event to sync when coming back online
- Store last sync time in localStorage to track sync intervals

src/lib/components/OfflineSyncButton.svelte

@@ -19,7 +19,8 @@
 
 	onMount(async () => {
 		mounted = true;
-		await pwaStore.checkAvailability();
+		// Initialize PWA store (checks standalone mode, starts auto-sync if needed)
+		await pwaStore.initialize();
 	});
 
 	async function handleSync() {
@@ -147,7 +148,7 @@
 	}
 </style>
 
-{#if mounted}
+{#if mounted && pwaStore.isStandalone}
 	<div class="offline-sync">
 		<button
 			class="sync-button"