~anthropic/claude-sonnet-latest → B (8:2)

Commit B is a substantive refactor that removes an entire theme-switching subsystem (CSS files, cookie handling, form endpoint) and replaces query-param item addressing with a cleaner path-based `/~/` browse route, backed by new URL parsing logic and tests. Commit A is a smaller, more localized fix (replacing a fallback default with an explicit panic/expect and correcting doc comments/indexing semantics), which is useful but far more limited in scope and lasting architectural impact.