feat: email notifications with tabbed channel UI (v0.9.10)

Add email as a second notification channel alongside ntfy:
- Tabbed channel selector: ntfy | Email | Telegram (coming soon) | Discord (coming soon)
- Active channel shown with green status dot on tab
- Email tab: address input, Save & Enable, Test, Disable — same UX pattern as ntfy
- Backend: SMTP config in settings (SMTP_HOST/PORT/USER/PASSWORD/FROM/STARTTLS)
- Dispatcher: _send_email() helper wired into dispatch_notifications
- POST /api/notifications/test/email endpoint with descriptive error messages
- Email fires in same window as ntfy (respects quiet hours / digest hold)
- Telegram and Discord tabs show coming-soon banners with planned feature description
- .env.example documents all SMTP settings with provider examples

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

frontend/lib/api.ts

@@ -249,6 +249,8 @@ export const notificationsAPI = {
     apiClient.post<NotificationTestResult>("/api/notifications/test/ntfy", data).then((r) => r.data),
   testRss: () =>
     apiClient.post<NotificationTestResult>("/api/notifications/test/rss").then((r) => r.data),
+  testEmail: () =>
+    apiClient.post<NotificationTestResult>("/api/notifications/test/email").then((r) => r.data),
   testFollowMode: (mode: string, event_type: string) =>
     apiClient.post<NotificationTestResult>("/api/notifications/test/follow-mode", { mode, event_type }).then((r) => r.data),
   getHistory: () =>

frontend/lib/types.ts

@@ -202,6 +202,8 @@ export interface NotificationSettings {
   ntfy_enabled: boolean;
   rss_enabled: boolean;
   rss_token: string | null;
+  email_enabled: boolean;
+  email_address: string;
   digest_enabled: boolean;
   digest_frequency: "daily" | "weekly";
   quiet_hours_start: number | null;