Add three browse modes to index page: alphabetical, by tag, by course

- Add SortMode type (AlphaSort | TagSort | CourseSort) to Roux.Html
- Routes: / (alpha), /sorted/tags, /sorted/course
- AlphaSort: recipes sorted by title (from metadata) or filename
- TagSort: recipes grouped under tag headings, empty state when none
- CourseSort: recipes grouped by course/category metadata
- Navigation bar in index page with active-highlighted buttons
- Add metaCourse field to Metadata model (+ emptyMetadata update)
- Add nubOrd helper for deduplicating tag/course lists
- All 26 tests passing, hlint clean

src/Roux/Server.hs

@@ -41,7 +41,13 @@ app recipeDir = do
 handleRequest :: [Idx.RecipeInfo] -> Wai.Application
 handleRequest recipes request respond =
     case Wai.pathInfo request of
-        [] -> respond (htmlResponse (Html.indexPage recipes))
+        -- Index page (default: alphabetical)
+        [] -> respond (htmlResponse (Html.indexPage Html.AlphaSort recipes))
+        -- Index page (sorted by tags)
+        ["sorted", "tags"] -> respond (htmlResponse (Html.indexPage Html.TagSort recipes))
+        -- Index page (sorted by course/category)
+        ["sorted", "course"] -> respond (htmlResponse (Html.indexPage Html.CourseSort recipes))
+        -- Recipe detail page
         ["recipes", rawPath] ->
             case lookupRecipe (urlDecodePath rawPath) recipes of
                 Just info -> respond (htmlResponse (Html.recipePage info))