Layout category store lists as horizontal columns

Each category is now a column side by side, so adding more
categories grows horizontally instead of vertically.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/client/src/routes/+page.svelte

@@ -127,9 +127,9 @@
 
     <!-- Store picker by category -->
     {#if storesByCategory().length > 0}
-      <div class="mt-6 space-y-4">
+      <div class="mt-6 flex gap-6 justify-center">
         {#each storesByCategory() as group}
-          <div>
+          <div class="min-w-0">
             <!-- Category header -->
             <button
               onclick={() => toggleCategory(group.category.id)}