Frontend handbook pages

React interface work, routes, components, client state, and browser behaviour. 3 pages.

1. 01Packbase webWhere Packbase browser behaviour lives and how routes, components, SDK state, stories, localisation, and the Worker fit together.
2. 02Admin and moderation workWhere Packbase staff and moderation features now belong, what already exists, and how to preserve server-enforced authority.
3. 03Frontend changesA repeatable Packbase web workflow for routes, components, SDK state, accessibility, localisation, and stories.