fix: honor urlStyle config for index and serve links

[EthanThatOneKid]

Summary

• \wiki serve\ reads \urlStyle\ from config when --style\ is omitted (aligned with \wiki build --url-style).
• \�uild_site\ resolves \url_style\ and \�ase_url\ from \WikiConfig\ when not passed explicitly.
• Index and page HTML helpers default to \DEFAULT_URL_STYLE\ (\dir) instead of \ile, so the All Pages list uses /wiki/page/\ links unless config says otherwise.

Test plan

• \ est_build_index_html_respects_url_style\
• \ est_index_links_use_config_file_url_style\
• Full \unittest discover\ passes