feat(hikes): view-transition flow across /hikes ↔ /hikes/[slug]

Cards + filter bar fly up from below when arriving at /hikes and drop
back down when leaving (in both directions of /hikes ↔ detail). Clicked
card morphs into the detail hero with a cross-fade so the thumbnail
dissolves into the map instead of snapping. Photo strip slides in from
the right. Root content cross-fades so metrics + content under the hero
phase in rather than appear at the end of the morph.

Track JSON moves from a client-side $effect into +page.ts so the strip
is in the DOM at view-transition snapshot time — also kills the brief
layout shift when it used to pop in post-load.

src/lib/components/hikes/HikeCard.svelte

@@ -53,7 +53,7 @@
 	const canton = $derived(resolveCanton(hike.canton));
 </script>
 
-<a class="card" href={resolve('/hikes/[slug]', { slug: hike.slug })} style="view-transition-name: hike-{hike.slug}">
+<a class="card" href={resolve('/hikes/[slug]', { slug: hike.slug })} style="view-transition-name: hike-{hike.slug}; view-transition-class: hike-fly-in">
 	<div class="cover">
 		{#if hike.cover.src}
 			<picture>