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.

apps/dashboard/src/lib/features/course/utils/settings-store.ts

@@ -15,6 +15,7 @@ type CourseSettings = {
   isContentGroupingEnabled: boolean;
   progressionMode: 'free' | 'sequential';
   callout: TCourseCallout | null;
+  welcomeEmailMessage: string;
 };
 
 export const settings = writable<CourseSettings>({
@@ -34,5 +35,6 @@ export const settings = writable<CourseSettings>({
   isPublished: false,
   isContentGroupingEnabled: true,
   progressionMode: 'free',
-  callout: null
+  callout: null,
+  welcomeEmailMessage: ''
 });