Minor improvements - normalize error strings and remove dead code - #27

Summary

Normalizes error strings to lowercase, wraps boundary errors, removes unused template helpers, and deduplicates TOML search-result wrapping. No user-facing behavior changes.

What's new

• Lowercase error strings - all fmt.Errorf strings in internal/book and cmd/book are lowercase with no trailing punctuation, including acronyms (url, not URL).
• Boundary error wrapping - web.LoadWebsite and bs.VerifyUniqueURL errors are wrapped with context at the call site in cmd/book/mark.go.
• Dead code removal - deletes unused UserViewTemplates and GetTemplate from internal/book/templates.go.
• TOML wrapping helper - adds catalog.SearchResultList and replaces anonymous struct wrappers in cmd/book/mark.go.

Behavior changes / guardrails

• No CLI behavior changes; existing commands, flags, and interactive flows produce the same output.
• Wrapped errors still chain correctly via %w.

Testing

• Existing unit tests cover error paths, search, and trash listing.
• make check runs fmt, vet, lint, and tests; all green.