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/docs/app/root.tsx

@@ -19,7 +19,7 @@ const DEFAULT_META_TITLE =
   'ClassroomIO Docs | Customer, Partner, and Employee Education';
 const DEFAULT_META_DESCRIPTION =
   'Learn how to create customer academies, partner certification programs, and employee training workflows in ClassroomIO.';
-const DEFAULT_OG_IMAGE_URL = 'https://brand.cdn.clsrio.com/og/classroomio-opengraph.png';
+const DEFAULT_OG_IMAGE_URL = 'https://brand.cdn.clsrio.com/og/classroomio-opengraph.jpg';
 const DEFAULT_OG_IMAGE_ALT = 'ClassroomIO documentation for customer, partner, and employee education';
 
 export const links: LinksFunction = () => [
@@ -60,7 +60,7 @@ export const meta: MetaFunction = () => [
   },
   {
     property: 'og:image:type',
-    content: 'image/png'
+    content: 'image/jpeg'
   },
   {
     property: 'og:image:width',
@@ -74,6 +74,31 @@ export const meta: MetaFunction = () => [
     property: 'og:image:secure_url',
     content: DEFAULT_OG_IMAGE_URL
   },
+  // WebP fallback for better compression on supported platforms
+  {
+    property: 'og:image',
+    content: 'https://brand.cdn.clsrio.com/og/classroomio-opengraph.webp'
+  },
+  {
+    property: 'og:image:alt',
+    content: DEFAULT_OG_IMAGE_ALT
+  },
+  {
+    property: 'og:image:type',
+    content: 'image/webp'
+  },
+  {
+    property: 'og:image:width',
+    content: '1920'
+  },
+  {
+    property: 'og:image:height',
+    content: '1080'
+  },
+  {
+    property: 'og:image:secure_url',
+    content: 'https://brand.cdn.clsrio.com/og/classroomio-opengraph.webp'
+  },
   {
     name: 'twitter:card',
     content: 'summary_large_image'