Decouple domain and storage code from TUI presentation

[polymorcodeus]

Summary

Adds a storage/UI config split and relocates all presentation concerns (spinners, styled errors, CLI renderers) out of internal/book, internal/catalog, and internal/web into cmd/book and internal/theme. Completes Phase 0 of the pkg/book extraction; CLI behavior is unchanged.

What's new

• theme.UIConfig - embeds *book.Config and carries runtime presentation state (Theme, Templates) plus LoadTheme, LoadTemplates, and StyledError; book.Config is now storage-only and internal/book imports stdlib + toml only.
• Spinner wrappers in cmd/book/spinner.go - loadCatalog and loadWebsite replace catalog.LoadCatalog and web.LoadWebsite; internal/catalog and internal/web no longer import huh or the spinner.
• CLI renderers in cmd/book/catalog.go - dumpDefaults and printConfigSources move out of internal/catalog as the book catalog theme|template|config implementations, ending the storage layer's theme dependency.
• book.MarshalCatalog - replaces PrintCatalog with a pure ([]byte, error) marshaler; cmd call sites print via the printCatalog helper.

Behavior changes / guardrails

• No user-facing changes: catalog config output and catalog theme/template dumps verified byte-identical against the previous binary.
• catalog.ResolveWritePath is newly exported so the relocated dumpDefaults keeps symlink-aware writes; CreateTOML shares the same resolution path.
• Storage-only functions (gc, doctor, migrate, indexCache) keep *book.Config signatures; cmd actions and TUI constructors take *theme.UIConfig and hand the embedded config to storage calls.

Testing

• New TestMarshalCatalog table test covers json, toml, and empty formats; TestUIConfigStyledError covers interactive and plain rendering.
• Smoke-tested shelf/collection/mark add, list, search, get, remove, restore, plus index rebuild, index sync, doctor, and gc against the previous binary.
• make check runs fmt, vet, lint, and tests; all green.