feat: notification digests via SMTP

Adds a periodic inbox-summary email per opted-in user. SMTP is
configured via appsettings (Smtp section); the digest is built from
existing analytics (health score, top senders, recommendations) and
sent by a new hourly DigestWorker, mirroring the GmailSyncWorker
pattern. Frequency and send hour are configurable (Digest section).

Backend: IEmailSender (SmtpEmailSender, MailKit), IDigestService,
DigestWorker, new User.DigestEnabled/LastDigestSentUtc fields +
migration, SettingsController for the per-user toggle and a
send-now test endpoint, all scoped to the authenticated UserId.

Frontend: SettingsApi + a bell/no-bell toggle button in the topbar.

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

frontend/src/components/Layout.jsx

@@ -4,6 +4,7 @@ import { AuthApi, SyncApi, AnalyticsApi } from '../api/client.js';
 import Logo from './Logo.jsx';
 import DevBanner from './DevBanner.jsx';
 import SyncStatus from './SyncStatus.jsx';
+import DigestToggle from './DigestToggle.jsx';
 import useSavedSearches from '../hooks/useSavedSearches.js';
 
 // ── Folder definitions ────────────────────────────────────────────────────────
@@ -255,6 +256,7 @@ export default function Layout() {
         </form>
         <div className="spacer" />
         <SyncStatus />
+        <DigestToggle />
         <button onClick={startSync}>Sync now</button>
         <span className="user">{user?.email}</span>
         <button className="ghost" onClick={logout}>Log out</button>