hub detail pages: shared Back button with proper single-history

#3 Back buttons: HubShell now renders a top-left "‹ Back" on each detail page
(/word /quote /onthisday). It mirrors the News reader's rule — if you arrived by
an in-app navigation, history.back() returns you to the hub with state/scroll
intact (and the browser Back button traverses the same single history); a direct
deep-link has no app history, so it falls back to goto('/home3'). Opt-out via
back={false}; relabel via backLabel.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/components/HubShell.svelte

@@ -1,14 +1,35 @@
 <script>
   // Shared shell for the hub + its detail pages (/word, /quote, /onthisday): the editorial
   // top bar, footer, fonts, and design tokens. Content goes in the default slot.
+  import { afterNavigate, goto } from '$app/navigation';
   import HubBar from './HubBar.svelte';
-  let { active = '', children } = $props();
+  let { active = '', back = true, backLabel = 'Back', children } = $props();
+
+  // Same single-history rule as the News reader's in-page Back: if we arrived here by an
+  // in-app navigation, history.back() returns to wherever (the hub) with state intact; a
+  // direct deep-link has no app history, so fall back to the hub.
+  let cameFromApp = $state(false);
+  afterNavigate(({ from }) => { if (from) cameFromApp = true; });
+  function goBack() {
+    if (cameFromApp && typeof history !== 'undefined') history.back();
+    else goto('/home3');
+  }
 </script>
 
 <div class="page">
   <HubBar {active} />
 
-  <main class="shell-main">{@render children?.()}</main>
+  <main class="shell-main">
+    {#if back}
+      <button class="back" onclick={goBack} aria-label="Go back">
+        <svg viewBox="0 0 24 24" width="15" height="15" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">
+          <path d="M15 18l-6-6 6-6" />
+        </svg>
+        {backLabel}
+      </button>
+    {/if}
+    {@render children?.()}
+  </main>
 
   <footer class="foot">upbeatBytes — no ads, no paywalls, no doomscrolling.</footer>
 </div>
@@ -29,9 +50,20 @@
 
   .shell-main {
     flex: 1; width: 100%; max-width: 1180px; margin: 0 auto; box-sizing: border-box;
-    padding: clamp(26px, 5vw, 56px) clamp(18px, 5vw, 44px) clamp(40px, 6vw, 72px);
+    padding: clamp(20px, 4vw, 40px) clamp(18px, 5vw, 44px) clamp(40px, 6vw, 72px);
   }
 
+  /* top-left Back — returns to the hub (or wherever you came from), preserving history */
+  .back {
+    display: inline-flex; align-items: center; gap: 6px; margin: 0 0 clamp(14px, 3vw, 24px);
+    background: none; border: none; cursor: pointer; padding: 6px 10px 6px 0;
+    font-family: inherit; font-size: 14px; font-weight: 600; color: var(--body);
+    transition: color 0.15s ease;
+  }
+  .back:hover { color: var(--teal); }
+  .back svg { transition: transform 0.15s ease; }
+  .back:hover svg { transform: translateX(-2px); }
+
   .foot {
     text-align: center; max-width: 1180px; width: 100%; margin: 14px auto 0; box-sizing: border-box;
     padding: 20px clamp(18px, 5vw, 44px) 30px; font-size: 13px; color: var(--muted);