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/components/exercise/edit-mode.svelte

@@ -53,7 +53,7 @@
     shouldDeleteExercise?: boolean;
     exerciseId: string;
     goBack?: () => void;
-    /** Self-paced + all auto-gradable question types: enforce points > 0 */
+    /** All auto-gradable question types (any course): enforce points > 0 for scoring */
     requiresPositivePointsForAutoGrade?: boolean;
     selfPacedCourse?: boolean;
     /** Public course restricts picker to auto-gradable question types only. */