feat(threejs): complete Three.js stack integration

- Add threejs.csv to src/data/stacks/ and cli/assets/data/stacks/
- Register all 14 stacks in STACK_CONFIG (was missing 9: react, nextjs,
  vue, svelte, astro, swiftui, nuxtjs, nuxt-ui, html-tailwind)
- Sync missing 9 stack CSVs from cli/assets to src/data/stacks/
- Update search.py docstring to list all 14 supported stacks
- Update all 15 platform templates: 13 stacks -> 14 stacks, add Three.js
- Remove empty threejs/ placeholder under templates/platforms/

Smoke test: 14/14 stack files resolve, threejs search returns 3 results