feat: live class sessions, CE workflow, and docs (#683)

Adds live-class session scheduling on lessons (join link, date/time,
timezone) with automatic 24h and 1h reminder emails plus a calendar
invite (.ics), a time-change notification with an updated invite, a
teacher-authored welcome email and session invite on enrollment, and a
"Notify students" quiz nudge.

LMS gets an upcoming live-sessions card and Join buttons, plus a
reusable live-session card component (3 states) in @cio/ui with a
Storybook story. The sidebar content tree now reflects sequential
progression locking like the content list, and the course-completion
confetti regression is fixed.

Org-based emails no longer reference ClassroomIO by name. Docs add
live-class, welcome-email, and course-progression guides, expand the
certificates guide (design, signatures, corrected labels), move
plan-required callouts to the top, flatten how-to-guides to the docs
root with redirects, and add screenshots. Includes an add-docs-image
skill.

packages/email/src/emails/teacher-student-buy-request.ts

@@ -2,6 +2,7 @@ import * as z from 'zod';
 
 import { defineEmail } from '../send';
 import { getDefaultTemplate } from '../templates';
+import { ZEmailBranding } from '../core/branding';
 
 export const teacherStudentBuyRequestEmail = defineEmail({
   id: 'teacherStudentBuyRequest',
@@ -9,7 +10,8 @@ export const teacherStudentBuyRequestEmail = defineEmail({
   schema: z.object({
     courseName: z.string().min(1),
     studentEmail: z.string().email(),
-    studentFullname: z.string().min(1)
+    studentFullname: z.string().min(1),
+    branding: ZEmailBranding
   }),
   render: (fields) => {
     const content = `
@@ -22,6 +24,6 @@ export const teacherStudentBuyRequestEmail = defineEmail({
       </p>
     `;
 
-    return getDefaultTemplate(content);
+    return getDefaultTemplate(content, fields.branding);
   }
 });