feat: AI categorisation fallback, unsubscribe confidence scoring, one-line summaries

Heuristic classifier falls back to AI only when it can't determine a category;
unsubscribe confidence blends method reliability with an AI safety opinion and
surfaces it in the Unsubscribe Manager; emails get an on-demand AI one-line
summary in the detail pane. All AI calls degrade gracefully when AI is disabled
or the provider errors.

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

frontend/src/pages/Senders.jsx

@@ -71,10 +71,13 @@ function SenderList({ senders, selectedId, onSelect, search, onSearch }) {
 function EmailDetail({ email: summary, onBack }) {
   const [detail, setDetail] = useState(null);
   const [loading, setLoading] = useState(true);
+  const [aiSummary, setAiSummary] = useState(null);
+  const [aiLoading, setAiLoading] = useState(false);
 
   useEffect(() => {
     setDetail(null);
     setLoading(true);
+    setAiSummary(null);
     EmailApi.get(summary.id)
       .then(setDetail)
       .catch(() => setDetail(null))
@@ -83,6 +86,14 @@ function EmailDetail({ email: summary, onBack }) {
 
   const email = detail ?? summary;
 
+  const fetchAiSummary = () => {
+    setAiLoading(true);
+    EmailApi.summary(summary.id)
+      .then((r) => setAiSummary(r.summary))
+      .catch(() => setAiSummary(null))
+      .finally(() => setAiLoading(false));
+  };
+
   return (
     <div className="sd-detail">
       <button className="sd-back" onClick={onBack}>← Back to list</button>
@@ -101,6 +112,18 @@ function EmailDetail({ email: summary, onBack }) {
         </div>
       </div>
 
+      {!loading && (
+        <div className="sd-ai-summary">
+          {aiSummary != null ? (
+            <div className="sd-ai-summary-text">✨ {aiSummary}</div>
+          ) : (
+            <button className="btn-sm" onClick={fetchAiSummary} disabled={aiLoading}>
+              {aiLoading ? 'Summarising…' : '✨ AI summary'}
+            </button>
+          )}
+        </div>
+      )}
+
       {loading && <div className="sd-body-loading muted">Loading message…</div>}
 
       {!loading && detail?.bodyText && (