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/components/Layout.jsx

@@ -3,6 +3,7 @@ import { useEffect, useState, useCallback } from 'react';
 import { AuthApi, SyncApi, AnalyticsApi } from '../api/client.js';
 import Logo from './Logo.jsx';
 import DevBanner from './DevBanner.jsx';
+import SyncStatus from './SyncStatus.jsx';
 
 // ── Folder definitions ────────────────────────────────────────────────────────
 
@@ -236,6 +237,7 @@ export default function Layout() {
           <button type="submit" className="search-btn" aria-label="Search">🔍</button>
         </form>
         <div className="spacer" />
+        <SyncStatus />
         <button onClick={startSync}>Sync now</button>
         <span className="user">{user?.email}</span>
         <button className="ghost" onClick={logout}>Log out</button>