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/src/base/button/button.svelte

@@ -24,6 +24,7 @@
       size: {
         default: 'ui:h-9 ui:px-4 ui:py-2 ui:has-[>svg]:px-3',
         sm: 'ui:h-8 ui:gap-1.5 ui:rounded-md ui:px-3 ui:has-[>svg]:px-2.5',
+        xs: 'ui:h-7 ui:gap-1 ui:rounded-md ui:px-2.5 ui:text-xs ui:has-[>svg]:px-2',
         lg: 'ui:h-10 ui:rounded-md ui:px-6 ui:has-[>svg]:px-4',
         icon: 'ui:size-9',
         'icon-2xs': 'ui:size-4',