fix(nav): restore DocumentTopBar back button sizing and add right padding

- BackButton gains showLabel prop: showLabel=false renders icon-only with
  aria-label, no mr-2 on svg (was causing 0px button width in topbar)
- DocumentTopBar: BackButton restored to h-11 w-11 circular touch target
  with showLabel=false matching the original 44×44px <a> it replaced
- Topbar row gets pr-4 (16px right padding per spec); action buttons div
  no longer needs its own pr-3

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

frontend/src/lib/components/BackButton.svelte

@@ -1,15 +1,16 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
-let { class: cls = 'mb-4' }: { class?: string } = $props();
+let { class: cls = 'mb-4', showLabel = true }: { class?: string; showLabel?: boolean } = $props();
 </script>
 
 <button
 	type="button"
 	onclick={() => history.back()}
+	aria-label={!showLabel ? m.btn_back() : undefined}
 	class="group {cls} inline-flex min-h-[44px] items-center text-xs font-bold tracking-widest text-ink-2 uppercase transition-colors outline-none hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring"
 >
 	<svg
-		class="mr-2 h-4 w-4 transform transition-transform group-hover:-translate-x-1"
+		class="{showLabel ? 'mr-2' : ''} h-4 w-4 transform transition-transform group-hover:-translate-x-1"
 		fill="none"
 		stroke="currentColor"
 		viewBox="0 0 24 24"
@@ -22,5 +23,5 @@ let { class: cls = 'mb-4' }: { class?: string } = $props();
 			d="M10 19l-7-7m0 0l7-7m-7 7h18"
 		/>
 	</svg>
-	{m.btn_back()}
+	{#if showLabel}{m.btn_back()}{/if}
 </button>