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/ui/README.md

@@ -96,6 +96,10 @@ Learner and author UIs for exercise questions (take, preview, review, submission
 
 Marketing / demo widget: left-hand list of question types and a live **take**-mode preview using `ExerciseQuestion.QuestionRenderer`. Copy is English-only (no dashboard i18n). Also consumed by the **`@cio/embeds`** app as a CDN bundle (`apps/embeds`).
 
+### Live session card (`src/custom/live-session-card/`)
+
+Presentational card for a live-class lesson with three states (`live`, `upcoming`, `ended`) derived from `lessonAt` + `durationMinutes`, or forced via the `status` prop (used by Storybook). Shows a join/copy action set when live, an "Add to calendar" combo button (Google, Outlook.com, Office 365, Yahoo, plus an `.ics` download for Apple) and a countdown when upcoming. All copy is passed in via the `labels: LiveSessionLabels` prop, so the dashboard wrapper supplies translated strings; `onCopyLink` fires after the link is copied (e.g. for a snackbar). See `Molecules/LiveSessionCard` in Storybook.
+
 ### Hooks (`src/hooks/`)
 
 Reusable Svelte hooks are located in the `src/hooks/` directory. These are Svelte 5 runes-based utilities that can be used across components.