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/ui/question/grade.svelte

@@ -16,6 +16,10 @@
       snackbar.error('grade cant be more than max value');
       grade = gradeMax;
     }
+    if (grade && grade < 0) {
+      snackbar.error('grade cant be less than 0');
+      grade = 0;
+    }
   });
 </script>
 
@@ -24,6 +28,7 @@
     placeholder={$t('course.navItem.lessons.exercises.new_exercise_modal.points')}
     bind:value={grade}
     max={gradeMax}
+    min={0}
     type="number"
     inputClassName="!w-16"
     isDisabled={disableGrading}