feat: sync status indicator, unsubscribe queue UI, email body preview

SyncStatus widget polls /sync/status and shows a live progress bar in
the topbar while syncing, or last-synced time / error otherwise.

Unsubscribe page reworked into a proper queue: status filter tabs
(All/Pending/Succeeded/Failed), select-all, sorted by volume, status
badges, and a result toast after processing.

GET /email/{id} returns full BodyText; Senders detail view fetches
and renders it instead of just the snippet.

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

frontend/src/pages/Senders.jsx

@@ -65,7 +65,21 @@ function SenderList({ senders, selectedId, onSelect, search, onSearch }) {
 
 // ── EmailDetail (full single-email view) ──────────────────────────────────────
 
-function EmailDetail({ email, onBack }) {
+function EmailDetail({ email: summary, onBack }) {
+  const [detail, setDetail] = useState(null);
+  const [loading, setLoading] = useState(true);
+
+  useEffect(() => {
+    setDetail(null);
+    setLoading(true);
+    EmailApi.get(summary.id)
+      .then(setDetail)
+      .catch(() => setDetail(null))
+      .finally(() => setLoading(false));
+  }, [summary.id]);
+
+  const email = detail ?? summary;
+
   return (
     <div className="sd-detail">
       <button className="sd-back" onClick={onBack}>← Back to list</button>
@@ -83,9 +97,17 @@ function EmailDetail({ email, onBack }) {
           )}
         </div>
       </div>
-      {email.snippet && (
+
+      {loading && <div className="sd-body-loading muted">Loading message…</div>}
+
+      {!loading && detail?.bodyText && (
+        <div className="sd-body">{detail.bodyText}</div>
+      )}
+
+      {!loading && !detail?.bodyText && email.snippet && (
         <div className="sd-snippet">{email.snippet}</div>
       )}
+
       <div className="sd-detail-actions">
         <button
           className="btn-sm"