feat: use authored category names for mods

Store categories as authored strings. Trim and deduplicate without changing case, compare without case sensitivity, and keep all observed categories available to filters and editors.

manifest.json

@@ -4,7 +4,7 @@
   "version": "0.1.0",
   "author": "ct",
   "description": "Clickable objects for picture scenes: a picture of its own over the background, a glow under the mouse, and a click that starts a scenario.",
-  "category": "engine, library",
+  "categories": ["engine", "library"],
   "depends": ["shell2d_base>=0.1.0", "ui_base>=0.1.0"],
   "sources": [ "src/**/*.cs" ],
   "ui": [],