Add YAML front matter parsing and use metadata titles in UI

- Add extractFrontMatter: parses YAML front matter (---...---) into
  a Metadata value, returns remaining body for Cooklang parsing
- Add parseYamlLines: extracts known keys (title, author, source,
  difficulty, cuisine, course, diet, tags, image, description)
  into canonical Metadata fields
- Add parseTagList: handles both [tag1, tag2] and tag1, tag2 formats
- Index page recipeItem now takes RecipeInfo and displays riTitle
  (from metadata) instead of raw filename
- All three sort modes (alpha, tag, course) use title-aware display
- OlivierSalad example shows its metadata title correctly

cooklang-examples/OlivierSalad.cook

@@ -1,5 +1,6 @@
 ---
 title: Olivier Salad
+tags: salad, side
 ---
 
 -- TODO add source