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/DocumentTopBar.svelte

@@ -129,12 +129,15 @@ let mobileMenuOpen = $state(false);
 
 <div data-topbar class="relative z-10 border-b border-line bg-surface shadow-sm">
 	<!-- Main row -->
-	<div class="flex h-[75px] shrink-0 items-center xs:h-[88px]">
+	<div class="flex h-[75px] shrink-0 items-center pr-4 xs:h-[88px]">
 		<!-- Accent bar -->
 		<div class="h-full w-[3px] shrink-0 bg-primary"></div>
 
 		<!-- Back button -->
-		<BackButton class="-ml-0.5 shrink-0" />
+		<BackButton
+			class="-ml-0.5 h-11 w-11 shrink-0 justify-center rounded-full hover:bg-muted"
+			showLabel={false}
+		/>
 
 		<!-- Divider -->
 		<div class="mx-2 h-6 w-px shrink-0 bg-line"></div>
@@ -190,7 +193,7 @@ let mobileMenuOpen = $state(false);
 		<div class="mx-3 hidden h-6 w-px shrink-0 bg-line md:block"></div>
 
 		<!-- Action buttons -->
-		<div class="flex shrink-0 items-center gap-1.5 pr-3 font-sans">
+		<div class="flex shrink-0 items-center gap-1.5 font-sans">
 			{#if canWrite && isPdf && !transcribeMode}
 				{@render transcribeBtn(false)}
 			{/if}