fix(export): render vimwiki-compatible HTML for templates, links, and tasks

Stock vimwiki templates and stylesheets rendered broken pages because the
HTML output diverged from vimwiki conventions on three fronts:

- Templates: the renderer only substituted nuwiki's `{{key}}` placeholders,
  so vimwiki's `%title%`/`%content%`/`%root_path%`/`%date%`/`%wiki_css%`
  passed through literally — dropping the body and breaking asset links.
  Both delimiter styles are now recognised; `%wiki_css%` aliases the css var.
- Links: `[[todo.wiki]]` exported to `todo.wiki.html`. render_page_html now
  takes the wiki file extension and strips it from wiki/interwiki targets
  (via index::strip_wiki_extension) before the `.html` URL is built, matching
  in-editor navigation. file:/local: links keep their literal extension.
- Tasks: checkbox items used bespoke `task-*` classes plus an `<input>`,
  which double-rendered against vimwiki stylesheets. Emit `done0..done4`
  (`[ ] [.] [o] [O] [X]`) and `rejected` (`[-]`) classes with no input — the
  stylesheet draws the box.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/nuwiki-lsp/src/commands.rs

@@ -3048,6 +3048,7 @@ pub mod export_ops {
             name,
             DiaryDate::today_utc(),
             &cfg.html,
+            Some(cfg.file_extension.as_str()),
             std::collections::HashMap::new(),
         )?;
         let out_path = output_path_for(&cfg.html, name);