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/api/src/services/jobs/email-jobs.ts

@@ -20,6 +20,8 @@ export interface EnqueueTemplateEmailInput<TId extends EmailId> extends CommonOp
   fields: z.infer<EmailSchemaFor<TId>>;
   from?: string;
   replyTo?: string;
+  /** Optional iCalendar (.ics) body attached as a text/calendar part. */
+  ics?: string;
 }
 
 export interface EnqueueRawEmailInput extends CommonOptions {
@@ -81,7 +83,8 @@ export async function enqueueTransactionalEmail<TId extends EmailId>(
         to: recipient,
         fields: validatedFields,
         from: input.from,
-        replyTo: input.replyTo
+        replyTo: input.replyTo,
+        ics: input.ics
       },
       { idempotencyKey: recipientKey(input.idempotencyKey, recipient, recipients.length) }
     );