refactor: migrate quiz-project, gstack, and superpowers to JS+JSDoc

Convert all 41 TypeScript source/config files in cc4e-course/quiz-project
(Next.js), gstack (Astro), and superpowers (Vite/React/Phaser) to plain
JavaScript with JSDoc type annotations, replacing tsconfig.json with
jsconfig.json (strict + checkJs) in each subproject. No behavior changes:
lint, typecheck, test, and build gates match their TypeScript baselines
exactly in all three subprojects. Update stack references in the affected
READMEs accordingly.

gstack/jsconfig.json

@@ -0,0 +1,13 @@
+{
+  "extends": "astro/tsconfigs/strict",
+  "compilerOptions": {
+    "baseUrl": ".",
+    "paths": {
+      "~/*": ["src/*"]
+    },
+    "allowJs": true,
+    "checkJs": true
+  },
+  "include": [".astro/types.d.ts", "**/*"],
+  "exclude": ["dist", "node_modules"]
+}