fix(#91): add px-4 base padding and fix admin tab overflow at 320px

Home and Admin had no horizontal padding below the sm breakpoint (640px),
causing content to bleed to viewport edges. Admin's flex justify-between
row with h1 + 4 tab buttons overflowed by ~110px at 320px.

- +page.svelte: add px-4 to <main> (sm:px-6 lg:px-8 unchanged)
- admin/+page.svelte: add px-4 to outer container; stack header row
  vertically on mobile (flex-col sm:flex-row); reduce tab button padding
  to px-2 on mobile (sm:px-4 on desktop)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -67,7 +67,7 @@ $effect(() => {
 });
 </script>
 
-<main class="mx-auto max-w-7xl py-8 font-sans sm:px-6 lg:px-8">
+<main class="mx-auto max-w-7xl px-4 py-8 font-sans sm:px-6 lg:px-8">
 	<SearchFilterBar
 		bind:q={q}
 		bind:from={from}