Dashboard core + walkthrough editor

Two sub-projects shipped back-to-back.

The dashboard got a real shell — a sidebar with workspace picker, sectioned nav, usage meter, and

shortcut hints. The library table gained filter tabs, debounced search, column sort, and 20-per-page

pagination with URL state.

The walkthrough editor was rebuilt with true versioning. Every publish forks a new version; the public

viewer points at the latest published version id. Library rows got bulk-select, the publish page moved

to its own sub-route, and every panel (Videos, Sizzle, Integrations, Analytics) got polish touches.