fix(ui): hide logo on mobile+tablet, fix admin tab overflow

- AppNav: hide entire logo div (incl. mr-10 margin) below md: breakpoint
  to eliminate the phantom whitespace left of the hamburger button
- admin: 2×2 grid on mobile → flex row at sm:, so "Schlagworte" fits

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

frontend/src/routes/admin/+page.svelte

@@ -16,7 +16,7 @@ let activeTab = $state('users');
 		<h1 class="font-serif text-3xl text-ink">{m.admin_heading()}</h1>
 
 		<!-- Tabs -->
-		<div class="flex rounded-lg border border-line bg-surface p-1 shadow-sm">
+		<div class="grid grid-cols-2 rounded-lg border border-line bg-surface p-1 shadow-sm sm:flex">
 			<button
 				class="rounded-md px-2 py-2 text-sm font-bold tracking-wide uppercase transition sm:px-4 {activeTab ===
 				'users'