feat: add per-article expand/collapse button when display is compact

When the slider is not at full (level < 4), each truncated article
shows a "Lire la suite" button to expand its summary inline.
A "Reduire" button collapses it back. Expansion is per-card and
independent of the global slider.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/SynthesisDetail.tsx

@@ -34,6 +34,17 @@ function truncateSummary(text: string, level: number): string {
 
 /** Renders a single news article with its title (linked) and summary. */
 const NewsItemCard: Component<{ item: NewsItemType; displayLevel: number }> = (props) => {
+  const [expanded, setExpanded] = createSignal(false);
+
+  const effectiveLevel = () => expanded() ? 4 : props.displayLevel;
+  const isTruncated = () => {
+    if (!props.item.summary || effectiveLevel() >= 4) return false;
+    if (effectiveLevel() === 1) return !!props.item.summary;
+    if (effectiveLevel() === 2) return props.item.summary.length > 160;
+    // level 3
+    return true;
+  };
+
   return (
     <div class="bg-white rounded-lg shadow-sm border border-gray-100 p-6 hover:shadow-md transition-shadow">
       <h3 class="text-lg font-semibold text-indigo-700 mb-2">
@@ -50,11 +61,27 @@ const NewsItemCard: Component<{ item: NewsItemType; displayLevel: number }> = (p
       <Show when={props.item.date}>
         <p class="text-xs text-gray-400 mb-1">{props.item.date}</p>
       </Show>
-      <Show when={props.displayLevel > 1 && props.item.summary}>
-        <p class={`text-gray-700 leading-relaxed text-sm ${props.displayLevel < 4 ? 'text-gray-500' : ''}`}>
-          {truncateSummary(props.item.summary, props.displayLevel)}
+      <Show when={effectiveLevel() > 1 && props.item.summary}>
+        <p class="text-gray-700 leading-relaxed text-sm">
+          {truncateSummary(props.item.summary, effectiveLevel())}
         </p>
       </Show>
+      <Show when={props.displayLevel < 4 && !expanded() && isTruncated()}>
+        <button
+          onClick={() => setExpanded(true)}
+          class="mt-2 text-xs text-indigo-600 hover:text-indigo-800 font-medium"
+        >
+          Lire la suite &darr;
+        </button>
+      </Show>
+      <Show when={expanded() && props.displayLevel < 4}>
+        <button
+          onClick={() => setExpanded(false)}
+          class="mt-2 text-xs text-indigo-600 hover:text-indigo-800 font-medium"
+        >
+          Reduire &uarr;
+        </button>
+      </Show>
     </div>
   );
 };