Extract HTML pattern detectors

[chubes4]

Summary

• Extract details, button, and navigation conversion paths into focused pattern classes.
• Keep HtmlTransformer output stable while reducing convertElement size.
• Preserve existing parity coverage.

Verification

• composer test
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Drafting and implementing the code/test changes.