Add Recently Added notebooks section to homepage

Server-rendered section between Featured and Explore showing the 4
most recently modified notebooks, excluding featured and untitled ones.
Compact card layout with engine badge, cell count, and relative date.

frontend/src/components/RecentNotebooks.astro

@@ -0,0 +1,92 @@
+---
+import { Icon } from 'astro-icon/components';
+import type { NotebookSummary } from '../lib/types';
+
+interface Props {
+  notebooks: NotebookSummary[];
+}
+
+const FEATURED_IDS = new Set([
+  'rc-lowpass-filter',
+  '555-astable-blinker',
+  'common-emitter-amplifier',
+]);
+
+const { notebooks } = Astro.props;
+
+const recent = notebooks
+  .filter((nb) => !FEATURED_IDS.has(nb.id) && nb.title !== 'Untitled Notebook')
+  .sort((a, b) => new Date(b.modified).getTime() - new Date(a.modified).getTime())
+  .slice(0, 4);
+
+function formatDate(iso: string): string {
+  try {
+    return new Date(iso).toLocaleDateString('en-US', {
+      month: 'short',
+      day: 'numeric',
+      timeZone: 'UTC',
+    });
+  } catch {
+    return iso;
+  }
+}
+
+const engineColor: Record<string, { border: string; bg: string; text: string }> = {
+  ngspice: { border: 'border-blue-500/30', bg: 'bg-blue-500/10', text: 'text-blue-400' },
+  ltspice: { border: 'border-amber-500/30', bg: 'bg-amber-500/10', text: 'text-amber-400' },
+};
+---
+
+{recent.length > 0 && (
+  <section class="max-w-6xl mx-auto px-6 py-12">
+    <div class="flex items-center gap-3 mb-6">
+      <Icon name="lucide:clock" class="w-4 h-4 text-slate-500" />
+      <h2 class="text-lg font-semibold text-slate-200">Recently Added</h2>
+    </div>
+
+    <div class="grid gap-4 sm:grid-cols-2 lg:grid-cols-4">
+      {recent.map((nb) => {
+        const colors = engineColor[nb.engine] ?? engineColor.ngspice;
+        return (
+          <a
+            href={`/notebook/${nb.id}`}
+            class:list={[
+              'group block rounded-lg border bg-slate-900/50 p-4',
+              'hover:border-blue-500/40 hover:bg-slate-800/70',
+              'hover:-translate-y-0.5 hover:shadow-md hover:shadow-black/20',
+              'transition-all duration-200',
+              'border-slate-800',
+            ]}
+          >
+            <div class="flex items-start justify-between mb-2">
+              <h3 class="text-sm font-semibold text-slate-100 group-hover:text-blue-400 transition-colors line-clamp-1">
+                {nb.title}
+              </h3>
+              <span class:list={[
+                'shrink-0 ml-2 inline-flex items-center px-1.5 py-0.5 rounded text-[10px] font-medium border',
+                colors.border, colors.bg, colors.text,
+              ]}>
+                {nb.engine}
+              </span>
+            </div>
+
+            {nb.description && (
+              <p class="text-xs text-slate-400 leading-relaxed mb-2 line-clamp-2">
+                {nb.description}
+              </p>
+            )}
+
+            <div class="flex items-center gap-2 text-[11px] text-slate-500">
+              <span class="inline-flex items-center gap-1">
+                <Icon name="lucide:layers" class="w-3 h-3" />
+                {nb.cell_count} cell{nb.cell_count !== 1 ? 's' : ''}
+              </span>
+              <span>&middot;</span>
+              <span>{formatDate(nb.modified)}</span>
+            </div>
+          </a>
+        );
+      })}
+    </div>
+  </section>
+)}

frontend/src/pages/index.astro

@@ -4,6 +4,7 @@ import NotebookLayout from '../layouts/NotebookLayout.astro';
 import NotebookGallery from '../components/NotebookGallery';
 import PipelineStrip from '../components/PipelineStrip.astro';
 import FeaturedNotebooks from '../components/FeaturedNotebooks.astro';
+import RecentNotebooks from '../components/RecentNotebooks.astro';
 import OscilloscopeDisplay from '../components/OscilloscopeDisplay.astro';
 import { fetchNotebookList } from '../lib/server-api';
 import type { NotebookSummary } from '../lib/types';
@@ -150,6 +151,11 @@ try {
     <FeaturedNotebooks />
   </section>
 
+  <!-- Recently Added -->
+  <section class="section-fade">
+    <RecentNotebooks notebooks={notebooks} />
+  </section>
+
   <!-- Notebook Gallery -->
   <section id="notebooks" class="max-w-6xl mx-auto px-6 py-20 scroll-mt-8">
     <div class="mb-8">