feat: add optional Vue 3 frontend integration

[h0pers]

Summary

• Add use_vue cookiecutter option for optional Vue 3 frontend with Composition API (JS only), shadcn-vue, Pinia, Vue Router, and django-vite
• Hybrid architecture: Django templates serve Vue SPA via {% vite_asset %} tags, catch-all URL pattern under /app/ for Vue Router, root / redirects to SPA
• Conditional Node.js Docker build stage (production only), CI matrix expansion (2x2x2), post-gen hook generates package-lock.json
• Polls app now shared between DRF and Vue options, with conditional file removal for each

Test plan

• Generate with use_drf=n use_vue=y — verify no api/ in polls, frontend present, Dockerfile has frontend-build stage
• Generate with use_drf=y use_vue=y — verify both api/ and templates/ in polls
• Generate with use_drf=n use_vue=n — verify no polls/ app, no frontend/
• Generate with use_drf=y use_vue=n — verify polls has api/ only, no frontend
• Run npm run dev in generated project frontend — verify Vite dev server starts
• Run npm run build — verify assets output to website/static/dist/
• Verify pre-commit passes for all combinations
• Verify Docker build succeeds for development and production targets