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/README.md

@@ -14,7 +14,7 @@ Scaffold only (v0.0.1.0). Modules are placeholders — landing page lists three
 npm install
 npm run dev          # http://localhost:4321/try-gstack/
 npm test         # Vitest (geom-engine unit tests)
-npm run typecheck    # Astro check + TS strict
+npm run typecheck    # Astro check + strict checkJs (JavaScript + JSDoc)
 npm run build        # Static output to dist/
 ```
 
@@ -31,7 +31,7 @@ Auto-deploys to GitHub Pages from `main` via `actions/deploy-pages@v4`. See `RUN
 - **Math rendering**: KaTeX, bundled locally + SSR-rendered — to be added with first canvas module
 - **Animation**: Web Animations API + `requestAnimationFrame` (no GSAP / Motion One / Lottie)
 - **Analytics**: Cloudflare Web Analytics, deferred until 50+ daily sessions
-- **i18n**: every string via `t()` from `src/i18n/vi.ts`; English added by adding `en.ts`
+- **i18n**: every string via `t()` from `src/i18n/vi.js`; English added by adding `en.js`
 
 ## License