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 - optimised query patterns, code-split curator forms, fixed cache invalidation gaps, and added missing database indexes. ISR revalidation reduced from 24h to 1h.

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.

Performance - optimised query patterns, code-split curator forms, fixed cache invalidation gaps, and added missing database indexes. ISR revalidation reduced from 24h to 1h.

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.