feat: add sequential progression and video watch lesson completion

Gate student content by completion order, track per-video watch progress
with seek enforcement, and auto-complete lessons at the configured
threshold. Show ring progress in the lesson header, keep watch progress
on rewatch, and sync progression locks client-side so the next item
unlocks after completion.

packages/storybook/src/templates/certificate/fixtures.ts

@@ -48,8 +48,8 @@ export function buildDesign(
     subtitle: overrides.subtitle ?? DEFAULT_CERTIFICATE_DESIGN.subtitle,
     descriptionOverride: overrides.descriptionOverride,
     signatories: overrides.signatories ?? [
-      { name: 'Margaret W. Holloway', role: 'Director of Studies' },
-      { name: 'James K. Reeves', role: 'Dean of Faculty' }
+      { name: 'Margaret W. Holloway', role: 'Director of Studies', enabled: true },
+      { name: 'James K. Reeves', role: 'Dean of Faculty', enabled: true }
     ],
     idFormat: overrides.idFormat ?? DEFAULT_CERTIFICATE_DESIGN.idFormat
   };