Dashboard sidebar, accessibility, and performance

Dashboard sidebar overhaul - collapsible curator and admin sections with animated transitions. Section state persists across navigations. The sidebar scrolls independently while the collapse button stays pinned.

Accessibility improvements - improved table and navigation screen reader support, better colour contrast ratios, keyboard navigation fixes across dialogs and menus, proper form label associations, and semantic list markup for entity grids.

Performance - tidied up several slow query patterns, split the heavier curator forms into smaller chunks, fixed places where edits weren't showing up immediately, and added missing database indexes. Cached pages refresh hourly instead of daily.

Legal compliance - self-service data export for UK GDPR, updated privacy policy and terms of use, copyright takedown procedure.

SEO - canonical URLs site-wide, OG images for series and universe pages, Twitter cards, and improved metadata across all pages.