fix: prevent hero image flash by aligning server/client random seed

Generate heroIndex on the server and pass it to the client so SSR and
hydration pick the same hero recipe, eliminating the image swap on
first interaction.

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

@@ -19,6 +19,7 @@ export const load: PageServerLoad = async ({ fetch, locals, params }) => {
     return {
         season: addFavoriteStatusToRecipes(item_season, userFavorites),
         all_brief: addFavoriteStatusToRecipes(item_all_brief, userFavorites),
-        session
+        session,
+        heroIndex: Math.random()
     };
 };

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

@@ -26,8 +26,8 @@
 	// Only recipes with hashed images (e.g. myrecipe.a1b2c3d4.webp)
 	const hasHashedImage = (r) => r.images?.length > 0 && /\.\w+\.\w+$/.test(r.images[0].mediapath);
 
-	// Pick once on mount — not reactive, so image and link always match
+	// Server-generated random index ensures SSR and client pick the same hero
-	const heroIndex = Math.random();
+	const heroIndex = data.heroIndex;
 	const heroRecipe = $derived.by(() => {
 		const seasonPool = data.season.filter(hasHashedImage);
 		const pool = seasonPool.length > 0 ? seasonPool : data.all_brief.filter(hasHashedImage);

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

@@ -34,6 +34,7 @@ export async function load({ data }) {
 					...data,
 					all_brief: rand_array(allBrief),
 					season: rand_array(seasonRecipes),
+					heroIndex: Math.random(),
 					isOffline: true
 				};
 			}