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/api/client.js

@@ -119,6 +119,7 @@ function folderToRequest(slug, page, pageSize) {
 }
 
 export const EmailApi = {
+  get:        (id) => api.get(`/email/${id}`).then((r) => r.data),
   markRead:   (id) => api.post(`/email/${id}/read`),
   markUnread: (id) => api.post(`/email/${id}/unread`),
   star:       (id) => api.post(`/email/${id}/star`),