Character meta editing for contributors, spoiler guard and SEO improvements

Book page polish - the reading status button below the cover image is now a single, cleaner link that matches the cover width. The segment count below the description now shows just the dominant type (e.g. "50 chapters" instead of "51 segments"). ISBNs have been removed from the public book page.

Character meta editing - a new edit button appears on character pages for contributors and curators. Clicking it opens a panel where you can update gender, species, origin, roles, and aliases. Curators save changes directly; contributors submit suggestions that go through the review process.

Character partial update fix - fixed a bug where editing one character field (like description) could accidentally clear other fields that weren't being changed.

Smarter contribute page - the /contribute page now shows different content based on whether you're signed in. Logged-out visitors see the recruitment pitch; signed-in contributors go straight to finding pages that need work.

Book description spoiler protection - book descriptions that may spoil earlier books are now protected with a blur and reveal button. Readers who haven't finished the book see the guard; those who have see the description normally.

SEO improvements - the homepage heading structure is improved, and search engine guidance is tightened up. Browse pages now include introductory text.

Accessibility fixes - the 404 page now meets touch-target size standards. Dropdown menus are capped at a sensible height to avoid overflowing the screen.

Copy fixes - "log in" changed to "sign in" across the site for consistency. Stray em dashes replaced with hyphens.

Curator cleanup page removed - the curator cleanup page has been removed. Any necessary cleanup is handled automatically.

Billy Bumbler species fix - corrected Oy's species entry (from the Dark Tower series) which had an incorrect format.