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/utils/src/validation/course/course.ts

@@ -364,6 +364,8 @@ const ZCourseMetadataFields = z.object({
   grading: z.boolean().optional(),
   lessonDownload: z.boolean().optional(),
   allowNewStudent: z.boolean(),
+  welcomeEmailMessage: z.string().max(20000).nullish(),
+  sessionTimezone: z.string().max(64).nullish(),
   sectionDisplay: z.record(z.string(), z.boolean()).optional(),
   isContentGroupingEnabled: z.boolean().optional(),
   progressionMode: z.enum(['free', 'sequential']).optional()

packages/utils/src/validation/mail/mail.ts

@@ -5,7 +5,9 @@ export const ZEmailData = z.object({
   to: z.email(),
   subject: z.string().min(1),
   content: z.string().min(1),
-  replyTo: z.string().optional()
+  replyTo: z.string().optional(),
+  /** Optional iCalendar (.ics) body attached as a text/calendar part. */
+  ics: z.string().optional()
 });
 export type TEmailData = z.infer<typeof ZEmailData>;