Add email notification on new lesson comment

Notify admins/curators (or configured notificationEmails) when a student
leaves a comment under a lesson. New setting notifyOnComment (default on) +
toggle in admin settings. Fires only for student comments, not admin/curator
replies; comment text is HTML-escaped in the email.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/admin/settings-form.tsx

@@ -274,6 +274,11 @@ export function SettingsForm({ initial }: { initial: Settings }) {
             checked={bool("notifyOnHomework")}
             onChange={(v) => set("notifyOnHomework", v ? "true" : "false")}
           />
+          <Toggle
+            label="Уведомлять о новом комментарии под уроком"
+            checked={bool("notifyOnComment")}
+            onChange={(v) => set("notifyOnComment", v ? "true" : "false")}
+          />
           <Toggle
             label="Уведомлять о новой регистрации ученика"
             checked={bool("notifyOnRegistration")}