feat: add PWA offline support for recipe pages

- Add service worker with caching for build assets, static files, images, and pages
- Add IndexedDB storage for recipes (brief and full data)
- Add offline-db API endpoint for bulk recipe download
- Add offline sync button component in header
- Add offline-shell page for direct navigation fallback
- Pre-cache __data.json for client-side navigation
- Add +page.ts universal load functions with IndexedDB fallback
- Add PWA manifest and icons for installability
- Update recipe page to handle missing data gracefully

src/routes/[recipeLang=recipeLang]/+page.ts

@@ -0,0 +1,50 @@
+import { browser } from '$app/environment';
+import { isOffline, canUseOfflineData } from '$lib/offline/helpers';
+import { getAllBriefRecipes, getBriefRecipesBySeason, isOfflineDataAvailable } from '$lib/offline/db';
+import { rand_array } from '$lib/js/randomize';
+
+export async function load({ data }) {
+	// On the server, just pass through the server data unchanged
+	if (!browser) {
+		return {
+			...data,
+			isOffline: false
+		};
+	}
+
+	// On the client, check if we need to load from IndexedDB
+	// This happens when:
+	// 1. We're offline (navigator.onLine is false)
+	// 2. Service worker returned offline flag
+	// 3. Server data is missing (e.g., client-side navigation while offline)
+	const shouldUseOfflineData = (isOffline() || data?.isOffline || !data?.all_brief?.length) && canUseOfflineData();
+
+	if (shouldUseOfflineData) {
+		try {
+			const hasOfflineData = await isOfflineDataAvailable();
+			if (hasOfflineData) {
+				const currentMonth = new Date().getMonth() + 1;
+
+				const [allBrief, seasonRecipes] = await Promise.all([
+					getAllBriefRecipes(),
+					getBriefRecipesBySeason(currentMonth)
+				]);
+
+				return {
+					...data,
+					all_brief: rand_array(allBrief),
+					season: rand_array(seasonRecipes),
+					isOffline: true
+				};
+			}
+		} catch (error) {
+			console.error('Failed to load offline data:', error);
+		}
+	}
+
+	// Return server data as-is
+	return {
+		...data,
+		isOffline: false
+	};
+}